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perlvar(1) - perlvar - Perl predefined variables - man 1 perlvar

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PERLVAR(1)             Perl Programmers Reference Guide             PERLVAR(1)



NAME
       perlvar - Perl predefined variables

DESCRIPTION
       Predefined Names

       The following names have special meaning to Perl.  Most punctuation
       names have reasonable mnemonics, or analogs in(1,8) the shells.  Neverthe-
       less(1,3), if(3,n) you wish to use long variable names, you need only say

           use English;

       at the top of your program. This aliases all the short names to the
       long names in(1,8) the current package. Some even have medium names, gener-
       ally borrowed from awk. In general, it's best to use the

           use English '-no_match_vars';

       invocation if(3,n) you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it
       avoids a certain performance hit with the use of regular expressions.
       See English.

       Variables that depend on the currently selected filehandle may be set(7,n,1 builtins)
       by calling an appropriate object method on the IO::Handle object,
       although this is less(1,3) efficient than using the regular built-in vari-
       ables. (Summary lines below for this contain the word HANDLE.) First
       you must say

           use IO::Handle;

       after which you may use either

           method HANDLE EXPR

       or more safely,

           HANDLE->method(EXPR)

       Each method returns the old value of the IO::Handle attribute.  The
       methods each take an optional EXPR, which, if(3,n) supplied, specifies the
       new value for the IO::Handle attribute in(1,8) question.  If not supplied,
       most methods do nothing to the current value--except for autoflush(),
       which will assume a 1 for you, just to be different.

       Because loading in(1,8) the IO::Handle class is an expensive operation, you
       should learn how to use the regular built-in variables.

       A few of these variables are considered "read-only".  This means that
       if(3,n) you try to assign to this variable, either directly or indirectly
       through a reference, you'll raise(3,n) a run-time exception.

       You should be very careful when modifying the default values of most
       special variables described in(1,8) this document. In most cases you want to
       localize these variables before changing them, since if(3,n) you don't, the
       change may affect other modules which rely on the default values of the
       special variables that you have changed. This is one of the correct
       ways to read(2,n,1 builtins) the whole file(1,n) at once:

           open(2,3,n) my $fh, "foo" or die $!;
           local $/; # enable localized slurp mode
           my $content = <$fh>;
           close(2,7,n) $fh;

       But the following code is quite bad:

           open(2,3,n) my $fh, "foo" or die $!;
           undef $/; # enable slurp mode
           my $content = <$fh>;
           close(2,7,n) $fh;

       since some other module, may want to read(2,n,1 builtins) data from some file(1,n) in(1,8) the
       default "line mode", so if(3,n) the code we have just presented has been
       executed, the global value of $/ is now changed for any other code run-
       ning inside the same Perl interpreter.

       Usually when a variable is localized you want to make sure that this
       change affects the shortest scope possible. So unless you are already
       inside some short "{}" block, you should create one yourself. For exam-
       ple:

           my $content = '';
           open(2,3,n) my $fh, "foo" or die $!;
           {
               local $/;
               $content = <$fh>;
           }
           close(2,7,n) $fh;

       Here is an example of how your own code can go broken:

           for (1..5){
               nasty_break();
               print "$_ ";
           }
           sub nasty_break {
               $_ = 5;
               # do something with $_
           }

       You probably expect this code to print:

           1 2 3 4 5

       but instead you get:

           5 5 5 5 5

       Why? Because nasty_break() modifies $_ without localizing it first. The
       fix is to add local():

               local $_ = 5;

       It's easy to notice the problem in(1,8) such a short example, but in(1,8) more
       complicated code you are looking for trouble if(3,n) you don't localize
       changes to the special variables.

       The following list is ordered by scalar variables first, then the
       arrays, then the hashes.

       $ARG
       $_      The default input and pattern-searching space.  The following
               pairs are equivalent:

                   while (<>) {...}    # equivalent only in(1,8) while!
                   while (defined($_ = <>)) {...}

                   /^Subject:/
                   $_ =~ /^Subject:/

                   tr/a-z/A-Z/
                   $_ =~ tr/a-z/A-Z/

                   chomp
                   chomp($_)

               Here are the places where Perl will assume $_ even if(3,n) you don't
               use it:

               *  Various unary functions, including functions like ord() and
                  int(), as well as the all file(1,n) tests ("-f", "-d") except for
                  "-t", which defaults to STDIN.

               *  Various list functions like print() and unlink(1,2)().

               *  The pattern matching operations "m//", "s///", and "tr///"
                  when used without an "=~" operator.

               *  The default iterator variable in(1,8) a "foreach" loop if(3,n) no
                  other variable is supplied.

               *  The implicit iterator variable in(1,8) the grep() and map() func-
                  tions.

               *  The default place to put an input record when a "<FH>" oper-
                  ation's result is tested by itself as the sole criterion of
                  a "while" test.  Outside a "while" test, this will not hap-
                  pen.

               (Mnemonic: underline is understood in(1,8) certain operations.)

       $a
       $b      Special package variables when using sort(1,3)(), see "sort(1,3)" in(1,8)
               perlfunc.  Because of this specialness $a and $b don't need to
               be declared (using use vars, or our()) even when using the
               "strict 'vars'" pragma.  Don't lexicalize them with "my $a" or
               "my $b" if(3,n) you want to be able to use them in(1,8) the sort(1,3)() com-
               parison block or function.

       $<digits>
               Contains the subpattern from the corresponding set(7,n,1 builtins) of capturing
               parentheses from the last pattern match, not counting patterns
               matched in(1,8) nested blocks that have been exited already.
               (Mnemonic: like \digits.)  These variables are all read-only
               and dynamically scoped to the current BLOCK.

       $MATCH
       $&      The string(3,n) matched by the last successful pattern match (not
               counting any matches hidden within a BLOCK or eval() enclosed
               by the current BLOCK).  (Mnemonic: like & in(1,8) some editors.)
               This variable is read-only and dynamically scoped to the cur-
               rent BLOCK.

               The use of this variable anywhere in(1,8) a program imposes a con-
               siderable performance penalty on all regular expression
               matches.  See "BUGS".

       $PREMATCH
       $`      The string(3,n) preceding whatever was matched by the last success-
               ful pattern match (not counting any matches hidden within a
               BLOCK or eval enclosed by the current BLOCK).  (Mnemonic: "`"
               often precedes a quoted string.)  This variable is read-only.

               The use of this variable anywhere in(1,8) a program imposes a con-
               siderable performance penalty on all regular expression
               matches.  See "BUGS".

       $POSTMATCH
       $'      The string(3,n) following whatever was matched by the last success-
               ful pattern match (not counting any matches hidden within a
               BLOCK or eval() enclosed by the current BLOCK).  (Mnemonic: "'"
               often follows a quoted string.)  Example:

                   local $_ = 'abcdefghi';
                   /def/;
                   print "$`:$&:$'\n";         # prints abc:def:ghi

               This variable is read-only and dynamically scoped to the cur-
               rent BLOCK.

               The use of this variable anywhere in(1,8) a program imposes a con-
               siderable performance penalty on all regular expression
               matches.  See "BUGS".

       $LAST_PAREN_MATCH
       $+      The text matched by the last bracket of the last successful
               search pattern.  This is useful if(3,n) you don't know which one of
               a set(7,n,1 builtins) of alternative patterns matched. For example:

                   /Version: (.*)|Revision: (.*)/ && ($rev = $+);

               (Mnemonic: be positive and forward looking.)  This variable is
               read-only and dynamically scoped to the current BLOCK.

       $^N     The text matched by the used group most-recently closed (i.e.
               the group with the rightmost closing parenthesis) of the last
               successful search pattern.  (Mnemonic: the (possibly) Nested
               parenthesis that most recently closed.)

               This is primarily used inside "(?{...})" blocks for examining
               text recently matched. For example, to effectively capture text
               to a variable (in(1,8) addition to $1, $2, etc.), replace "(...)"
               with

                    (?:(...)(?{ $var = $^N }))

               By setting and then using $var in(1,8) this way relieves you from
               having to worry about exactly which numbered set(7,n,1 builtins) of parentheses
               they are.

               This variable is dynamically scoped to the current BLOCK.

       @LAST_MATCH_END
       @+      This array holds the offsets of the ends of the last successful
               submatches in(1,8) the currently active dynamic scope.  $+[0] is the
               offset into the string(3,n) of the end of the entire match.  This is
               the same value as what the "pos" function returns when called
               on the variable that was matched against.  The nth element of
               this array holds the offset of the nth submatch, so $+[1] is
               the offset past where $1 ends, $+[2] the offset past where $2
               ends, and so on.  You can use $#+ to determine how many sub-
               groups were in(1,8) the last successful match.  See the examples
               given for the "@-" variable.

       $*      Set to a non-zero integer value to do multi-line matching
               within a string(3,n), 0 (or undefined) to tell Perl that it can
               assume that strings contain a single line, for the purpose of
               optimizing pattern matches.  Pattern matches on strings con-
               taining multiple newlines can produce confusing results when $*
               is 0 or undefined. Default is undefined.  (Mnemonic: * matches
               multiple things.) This variable influences the interpretation
               of only "^" and "$". A literal newline can be searched for even
               when "$* == 0".

               Use of $* is deprecated in(1,8) modern Perl, supplanted by the "/s"
               and "/m" modifiers on pattern matching.

               Assigning a non-numerical value to $* triggers a warning (and
               makes $* act if(3,n) "$* == 0"), while assigning a numerical value
               to $* makes that an implicit "int" is applied on the value.

       HANDLE->input_line_number(EXPR)
       $INPUT_LINE_NUMBER
       $NR
       $.      Current line number for the last filehandle accessed.

               Each filehandle in(1,8) Perl counts the number of lines that have
               been read(2,n,1 builtins) from it.  (Depending on the value of $/, Perl's idea
               of what constitutes a line may not match yours.)  When a line
               is read(2,n,1 builtins) from a filehandle (via readline() or "<>"), or when
               tell() or seek() is called on it, $. becomes an alias to the
               line counter for that filehandle.

               You can adjust the counter by assigning to $., but this will
               not actually move(3x,7,3x curs_move) the seek pointer.  Localizing $. will not
               localize the filehandle's line count.  Instead, it will local-
               ize perl's notion of which filehandle $. is currently aliased
               to.

               $. is reset(1,7,1 tput) when the filehandle is closed, but not when an open(2,3,n)
               filehandle is reopened without an intervening close(2,7,n)().  For
               more details, see "I/O Operators" in(1,8) perlop.  Because "<>"
               never does an explicit close(2,7,n), line numbers increase across ARGV
               files (but see examples in(1,8) "eof" in(1,8) perlfunc).

               You can also use "HANDLE->input_line_number(EXPR)" to access(2,5)
               the line counter for a given filehandle without having to worry
               about which handle you last accessed.

               (Mnemonic: many programs use "." to mean the current line num-
               ber.)

       IO::Handle->input_record_separator(EXPR)
       $INPUT_RECORD_SEPARATOR
       $RS
       $/      The input record separator, newline by default.  This influ-
               ences Perl's idea of what a "line" is.  Works like awk's RS
               variable, including treating empty lines as a terminator if(3,n) set(7,n,1 builtins)
               to the null string.  (An empty line cannot contain any spaces
               or tabs.)  You may set(7,n,1 builtins) it to a multi-character string(3,n) to match
               a multi-character terminator, or to "undef" to read(2,n,1 builtins) through the
               end of file.  Setting it to "\n\n" means something slightly
               different than setting to "", if(3,n) the file(1,n) contains consecutive
               empty lines.  Setting to "" will treat two or more consecutive
               empty lines as a single empty line.  Setting to "\n\n" will
               blindly assume that the next input character belongs to the
               next paragraph, even if(3,n) it's a newline.  (Mnemonic: / delimits
               line boundaries when quoting poetry.)

                   local $/;           # enable "slurp" mode
                   local $_ = <FH>;    # whole file(1,n) now here
                   s/\n[ \t]+/ /g;

               Remember: the value of $/ is a string(3,n), not a regex.  awk has to
               be better for something. :-)

               Setting $/ to a reference to an integer, scalar containing an
               integer, or scalar that's convertible to an integer will
               attempt to read(2,n,1 builtins) records instead of lines, with the maximum
               record size being the referenced integer.  So this:

                   local $/ = \32768; # or \"32768", or \$var_containing_32768
                   open(2,3,n) my $fh, $myfile or die $!;
                   local $_ = <$fh>;

               will read(2,n,1 builtins) a record of no more than 32768 bytes from FILE.  If
               you're not reading from a record-oriented file(1,n) (or your OS
               doesn't have record-oriented files), then you'll likely get a
               full chunk of data with every read.  If a record is larger than
               the record size you've set(7,n,1 builtins), you'll get the record back in(1,8)
               pieces.

               On VMS, record reads are done with the equivalent of "sysread",
               so it's best not to mix record and non-record reads on the same
               file.  (This is unlikely to be a problem, because any file(1,n)
               you'd want to read(2,n,1 builtins) in(1,8) record mode is probably unusable in(1,8) line
               mode.)  Non-VMS systems do normal I/O, so it's safe to mix
               record and non-record reads of a file.

               See also "Newlines" in(1,8) perlport.  Also see $..

       HANDLE->autoflush(EXPR)
       $OUTPUT_AUTOFLUSH
       $|      If set(7,n,1 builtins) to nonzero, forces a flush(8,n) right away and after every
               write(1,2) or print on the currently selected output channel.
               Default is 0 (regardless of whether the channel is really
               buffered by the system or not; $| tells you only whether you've
               asked Perl explicitly to flush(8,n) after each write(1,2)).  STDOUT will
               typically be line buffered if(3,n) output is to the terminal and
               block buffered otherwise.  Setting this variable is useful pri-
               marily when you are outputting to a pipe(2,8) or socket(2,7,n), such as
               when you are running a Perl program under rsh and want to see
               the output as it's happening.  This has no effect on input
               buffering.  See "getc" in(1,8) perlfunc for that.  (Mnemonic: when
               you want your pipes to be piping hot.)

       IO::Handle->output_field_separator EXPR
       $OUTPUT_FIELD_SEPARATOR
       $OFS
       $,      The output field separator for the print operator.  Ordinarily
               the print operator simply prints out its arguments without fur-
               ther adornment.  To get behavior more like awk, set(7,n,1 builtins) this vari-
               able as you would set(7,n,1 builtins) awk's OFS variable to specify what is
               printed between fields.  (Mnemonic: what is printed when there
               is a "," in(1,8) your print statement.)

       IO::Handle->output_record_separator EXPR
       $OUTPUT_RECORD_SEPARATOR
       $ORS
       $\      The output record separator for the print operator.  Ordinarily
               the print operator simply prints out its arguments as is, with
               no trailing newline or other end-of-record string(3,n) added.  To
               get behavior more like awk, set(7,n,1 builtins) this variable as you would set(7,n,1 builtins)
               awk's ORS variable to specify what is printed at the end of the
               print.  (Mnemonic: you set(7,n,1 builtins) "$\" instead of adding "\n" at the
               end of the print.  Also, it's just like $/, but it's what you
               get "back" from Perl.)

       $LIST_SEPARATOR
       $"      This is like $, except that it applies to array and slice val-
               ues interpolated into a double-quoted string(3,n) (or similar inter-
               preted string(3,n)).  Default is a space.  (Mnemonic: obvious, I
               think.)

       $SUBSCRIPT_SEPARATOR
       $SUBSEP
       $;      The subscript separator for multidimensional array emulation.
               If you refer to a hash element as

                   $foo{$a,$b,$c}

               it really means

                   $foo{join(1,n)($;, $a, $b, $c)}

               But don't put

                   @foo{$a,$b,$c}      # a slice--note the @

               which means

                   ($foo{$a},$foo{$b},$foo{$c})

               Default is "\034", the same as SUBSEP in(1,8) awk.  If your keys
               contain binary data there might not be any safe value for $;.
               (Mnemonic: comma (the syntactic subscript separator) is a
               semi-semicolon.  Yeah, I know, it's pretty lame, but $, is
               already taken for something more important.)

               Consider using "real" multidimensional arrays as described in(1,8)
               perllol.

       $#      The output format for printed numbers.  This variable is a
               half-hearted attempt to emulate awk's OFMT variable.  There are
               times, however, when awk and Perl have differing notions of
               what counts as numeric.  The initial value is "%.ng", where n
               is the value of the macro DBL_DIG from your system's float.h.
               This is different from awk's default OFMT setting of "%.6g", so
               you need to set(7,n,1 builtins) $# explicitly to get awk's value.  (Mnemonic: #
               is the number sign.)

               Use of $# is deprecated.

       HANDLE->format_page_number(EXPR)
       $FORMAT_PAGE_NUMBER
       $%      The current page number of the currently selected output chan-
               nel.  Used with formats.  (Mnemonic: % is page number in(1,8)
               nroff.)

       HANDLE->format_lines_per_page(EXPR)
       $FORMAT_LINES_PER_PAGE
       $=      The current page length (printable lines) of the currently
               selected output channel.  Default is 60.  Used with formats.
               (Mnemonic: = has horizontal lines.)

       HANDLE->format_lines_left(EXPR)
       $FORMAT_LINES_LEFT
       $-      The number of lines left on the page of the currently selected
               output channel.  Used with formats.  (Mnemonic: lines_on_page -
               lines_printed.)

       @LAST_MATCH_START
       @-      $-[0] is the offset of the start of the last successful match.
               "$-["n"]" is the offset of the start of the substring matched
               by n-th subpattern, or undef if(3,n) the subpattern did not match.

               Thus after a match against $_, $& coincides with "substr $_,
               $-[0], $+[0] - $-[0]".  Similarly, "$"n coincides with "substr
               $_, $-["n"], $+["n"] - $-["n"]" if(3,n) "$-["n"]" is defined, and $+
               coincides with "substr $_, $-[$#-], $+[$#-]".  One can use
               "$#-" to find the last matched subgroup in(1,8) the last successful
               match.  Contrast with $#+, the number of subgroups in(1,8) the regu-
               lar expression.  Compare with "@+".

               This array holds the offsets of the beginnings of the last suc-
               cessful submatches in(1,8) the currently active dynamic scope.
               "$-[0]" is the offset into the string(3,n) of the beginning of the
               entire match.  The nth element of this array holds the offset
               of the nth submatch, so "$-[1]" is the offset where $1 begins,
               "$-[2]" the offset where $2 begins, and so on.

               After a match against some variable $var:

               $` is the same as "substr($var, 0, $-[0])"
               $& is the same as "substr($var, $-[0], $+[0] - $-[0])"
               $' is the same as "substr($var, $+[0])"
               $1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
               $2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
               $3 is the same as "substr $var, $-[3], $+[3] - $-[3])"
       HANDLE->format_name(EXPR)
       $FORMAT_NAME
       $~      The name of the current report format for the currently
               selected output channel.  Default is the name of the filehan-
               dle.  (Mnemonic: brother to $^.)

       HANDLE->format_top_name(EXPR)
       $FORMAT_TOP_NAME
       $^      The name of the current top-of-page format for the currently
               selected output channel.  Default is the name of the filehandle
               with _TOP appended.  (Mnemonic: points to top of page.)

       IO::Handle->format_line_break_characters EXPR
       $FORMAT_LINE_BREAK_CHARACTERS
       $:      The current set(7,n,1 builtins) of characters after which a string(3,n) may be bro-
               ken to fill continuation fields (starting with ^) in(1,8) a format.
               Default is " \n-", to break on whitespace or hyphens.
               (Mnemonic: a "colon" in(1,8) poetry is a part of a line.)

       IO::Handle->format_formfeed EXPR
       $FORMAT_FORMFEED
       $^L     What formats output as a form feed.  Default is \f.

       $ACCUMULATOR
       $^A     The current value of the write(1,2)() accumulator for format()
               lines.  A format contains formline() calls that put their
               result into $^A.  After calling its format, write(1,2)() prints out
               the contents of $^A and empties.  So you never really see the
               contents of $^A unless you call formline() yourself and then
               look(1,8,3 Search::Dict) at it.  See perlform and "formline()" in(1,8) perlfunc.

       $CHILD_ERROR
       $?      The status returned by the last pipe(2,8) close(2,7,n), backtick (``) com-
               mand, successful call to wait() or waitpid(), or from the sys-
               tem() operator.  This is just the 16-bit status word returned
               by the wait() system call (or else is made up to look(1,8,3 Search::Dict) like it).
               Thus, the exit(3,n,1 builtins) value of the subprocess is really ("$? >> 8"),
               and "$? & 127" gives which signal(2,7), if(3,n) any, the process died
               from, and "$? & 128" reports whether there was a core dump.
               (Mnemonic: similar to sh and ksh.)

               Additionally, if(3,n) the "h_errno" variable is supported in(1,8) C, its
               value is returned via $? if(3,n) any "gethost*()" function fails.

               If you have installed a signal(2,7) handler for "SIGCHLD", the value
               of $? will usually be wrong outside that handler.

               Inside an "END" subroutine $? contains the value that is going
               to be given to "exit(3,n,1 builtins)()".  You can modify $? in(1,8) an "END" subrou-
               tine to change the exit(3,n,1 builtins) status of your program.  For example:

                   END {
                       $? = 1 if(3,n) $? == 255;  # die would make it 255
                   }

               Under VMS, the pragma "use vmsish 'status'" makes $? reflect
               the actual VMS exit(3,n,1 builtins) status, instead of the default emulation of
               POSIX status; see "$?" in(1,8) perlvms for details.

               Also see "Error Indicators".

       ${^ENCODING}
               The object reference to the Encode object that is used to con-
               vert the source code to Unicode.  Thanks to this variable your
               perl script does not have to be written in(1,8) UTF-8.  Default is
               undef.  The direct manipulation of this variable is highly dis-
               couraged.  See encoding(3,n) for more details.

       $OS_ERROR
       $ERRNO
       $!      If used numerically, yields the current value of the C "errno"
               variable, or in(1,8) other words, if(3,n) a system or library call fails,
               it sets this variable.  This means that the value of $! is
               meaningful only immediately after a failure:

                   if(3,n) (open(2,3,n)(FH, $filename)) {
                       # Here $! is meaningless.
                       ...
                   } else {
                       # ONLY here is $! meaningful.
                       ...
                       # Already here $! might be meaningless.
                   }
                   # Since here we might have either success or failure,
                   # here $! is meaningless.

               In the above meaningless stands for anything: zero, non-zero,
               "undef".  A successful system or library call does not set(7,n,1 builtins) the
               variable to zero.

               If used as a string(3,n), yields the corresponding system error(8,n)
               string.  You can assign a number to $! to set(7,n,1 builtins) errno if(3,n), for
               instance, you want "$!" to return the string(3,n) for error(8,n) n, or
               you want to set(7,n,1 builtins) the exit(3,n,1 builtins) value for the die() operator.
               (Mnemonic: What just went bang?)

               Also see "Error Indicators".

       %!      Each element of "%!" has a true value only if(3,n) $! is set(7,n,1 builtins) to that
               value.  For example, $!{ENOENT} is true if(3,n) and only if(3,n) the cur-
               rent value of $! is "ENOENT"; that is, if(3,n) the most recent error(8,n)
               was "No such file(1,n) or directory" (or its moral equivalent: not
               all operating systems give that exact error(8,n), and certainly not
               all languages).  To check if(3,n) a particular key is meaningful on
               your system, use "exists $!{the_key}"; for a list of legal
               keys, use "keys %!".  See Errno for more information, and also
               see above for the validity of $!.

       $EXTENDED_OS_ERROR
       $^E     Error information specific to the current operating system.  At
               the moment, this differs from $! under only VMS, OS/2, and
               Win32 (and for MacPerl).  On all other platforms, $^E is always
               just the same as $!.

               Under VMS, $^E provides the VMS status value from the last sys-
               tem error.  This is more specific information about the last
               system error(8,n) than that provided by $!.  This is particularly
               important when $! is set(7,n,1 builtins) to EVMSERR.

               Under OS/2, $^E is set(7,n,1 builtins) to the error(8,n) code of the last call to
               OS/2 API either via CRT, or directly from perl.

               Under Win32, $^E always returns the last error(8,n) information
               reported by the Win32 call "GetLastError()" which describes the
               last error(8,n) from within the Win32 API.  Most Win32-specific code
               will report errors via $^E.  ANSI C and Unix-like calls set(7,n,1 builtins)
               "errno" and so most portable Perl code will report errors via
               $!.

               Caveats mentioned in(1,8) the description of $! generally apply to
               $^E, also.  (Mnemonic: Extra error(8,n) explanation.)

               Also see "Error Indicators".

       $EVAL_ERROR
       $@      The Perl syntax error(8,n) message from the last eval() operator.
               If $@ is the null string(3,n), the last eval() parsed and executed
               correctly (although the operations you invoked may have failed
               in(1,8) the normal fashion).  (Mnemonic: Where was the syntax error(8,n)
               "at"?)

               Warning messages are not collected in(1,8) this variable.  You can,
               however, set(7,n,1 builtins) up a routine to process warnings by setting
               $SIG{__WARN__} as described below.

               Also see "Error Indicators".

       $PROCESS_ID
       $PID
       $$      The process number of the Perl running this script.  You should
               consider this variable read-only, although it will be altered
               across fork() calls.  (Mnemonic: same as shells.)

               Note for Linux users: on Linux, the C functions "getpid()" and
               "getppid()" return different values from different threads. In
               order to be portable, this behavior is not reflected by $$,
               whose value remains consistent across threads. If you want to
               call the underlying "getpid()", you may use the CPAN module
               "Linux::Pid".

       $REAL_USER_ID
       $UID
       $<      The real uid of this process.  (Mnemonic: it's the uid you came
               from, if(3,n) you're running setuid.)  You can change both the real
               uid and the effective uid at the same time(1,2,n) by using
               POSIX::setuid().

       $EFFECTIVE_USER_ID
       $EUID
       $>      The effective uid of this process.  Example:

                   $< = $>;            # set(7,n,1 builtins) real to effective uid
                   ($<,$>) = ($>,$<);  # swap real and effective uid

               You can change both the effective uid and the real uid at the
               same time(1,2,n) by using POSIX::setuid().

               (Mnemonic: it's the uid you went to, if(3,n) you're running setuid.)
               $< and $> can be swapped only on machines supporting
               setreuid().

       $REAL_GROUP_ID
       $GID
       $(      The real gid of this process.  If you are on a machine that
               supports membership in(1,8) multiple groups simultaneously, gives a
               space separated list of groups you are in.  The first number is
               the one returned by getgid(), and the subsequent ones by get-
               groups(), one of which may be the same as the first number.

               However, a value assigned to $( must be a single number used to
               set(7,n,1 builtins) the real gid.  So the value given by $( should not be
               assigned back to $( without being forced numeric, such as by
               adding zero.

               You can change both the real gid and the effective gid at the
               same time(1,2,n) by using POSIX::setgid().

               (Mnemonic: parentheses are used to group things.  The real gid
               is the group you left, if(3,n) you're running setgid.)

       $EFFECTIVE_GROUP_ID
       $EGID
       $)      The effective gid of this process.  If you are on a machine
               that supports membership in(1,8) multiple groups simultaneously,
               gives a space separated list of groups you are in.  The first
               number is the one returned by getegid(), and the subsequent
               ones by getgroups(), one of which may be the same as the first
               number.

               Similarly, a value assigned to $) must also be a space-sepa-
               rated list of numbers.  The first number sets the effective
               gid, and the rest (if(3,n) any) are passed to setgroups().  To get
               the effect of an empty list for setgroups(), just repeat the
               new effective gid; that is, to force an effective gid of 5 and
               an effectively empty setgroups() list, say " $) = "5 5" ".

               You can change both the effective gid and the real gid at the
               same time(1,2,n) by using POSIX::setgid() (use only a single numeric
               argument).

               (Mnemonic: parentheses are used to group things.  The effective
               gid is the group that's right for you, if(3,n) you're running set-
               gid.)

               $<, $>, $( and $) can be set(7,n,1 builtins) only on machines that support the
               corresponding set(7,n,1 builtins)[re][ug]id() routine.  $( and $) can be
               swapped only on machines supporting setregid().

       $PROGRAM_NAME
       $0      Contains the name of the program being executed.

               On some (read: not all) operating systems assigning to $0 modi-
               fies the argument area that the "ps" program sees.  On some
               platforms you may have to use special "ps" options or a differ-
               ent "ps" to see the changes.  Modifying the $0 is more useful
               as a way of indicating the current program state than it is for
               hiding the program you're running.  (Mnemonic: same as sh and
               ksh.)

               Note that there are platform specific limitations on the the
               maximum length of $0.  In the most extreme case it may be lim-
               ited to the space occupied by the original $0.

               In some platforms there may be arbitrary amount of padding, for
               example space characters, after the modified name as shown by
               "ps".  In some platforms this padding may extend all the way to
               the original length of the argument area, no matter what you do
               (this is the case for example with Linux 2.2).

               Note for BSD users: setting $0 does not completely remove
               "perl" from the ps(1) output.  For example, setting $0 to "foo-
               bar" may result in(1,8) "perl: foobar (perl)" (whether both the
               "perl: " prefix and the " (perl)" suffix are shown depends on
               your exact BSD variant and version(1,3,5)).  This is an operating sys-
               tem feature, Perl cannot help it.

               In multithreaded scripts Perl coordinates the threads so that
               any thread may modify its copy of the $0 and the change becomes
               visible to ps(1) (assuming the operating system plays along).
               Note that the the view of $0 the other threads have will not
               change since they have their own copies of it.

       $[      The index of the first element in(1,8) an array, and of the first
               character in(1,8) a substring.  Default is 0, but you could theoret-
               ically set(7,n,1 builtins) it to 1 to make Perl behave more like awk (or For-
               tran) when subscripting and when evaluating the index() and
               substr() functions.  (Mnemonic: [ begins subscripts.)

               As of release 5 of Perl, assignment to $[ is treated as a com-
               piler directive, and cannot influence the behavior of any other
               file.  (That's why you can only assign compile-time constants
               to it.)  Its use is highly discouraged.

               Note that, unlike other compile-time directives (such as
               strict), assignment to $[ can be seen from outer lexical scopes
               in(1,8) the same file.  However, you can use local() on it to
               strictly bound its value to a lexical block.

       $]      The version(1,3,5) + patchlevel / 1000 of the Perl interpreter.  This
               variable can be used to determine whether the Perl interpreter
               executing a script is in(1,8) the right range of versions.
               (Mnemonic: Is this version(1,3,5) of perl in(1,8) the right bracket?)
               Example:

                   warn "No checksumming!\n" if(3,n) $] < 3.019;

               See also the documentation of "use VERSION" and "require VER-
               SION" for a convenient way to fail if(3,n) the running Perl inter-
               preter is too old.

               When testing the variable, to steer clear(1,3x,3x clrtobot) of floating point
               inaccuracies you might want to prefer the inequality tests "<"
               and ">" to the tests containing equivalence: "<=", "==", and
               ">=".

               The floating point representation can sometimes lead to inaccu-
               rate numeric comparisons.  See $^V for a more modern represen-
               tation of the Perl version(1,3,5) that allows accurate string(3,n) compar-
               isons.

       $COMPILING
       $^C     The current value of the flag associated with the -c switch.
               Mainly of use with -MO=... to allow code to alter its behavior
               when being compiled, such as for example to AUTOLOAD at compile
               time(1,2,n) rather than normal, deferred loading.  See perlcc.  Set-
               ting "$^C = 1" is similar to calling "B::minus_c".

       $DEBUGGING
       $^D     The current value of the debugging flags.  (Mnemonic: value of
               -D switch.) May be read(2,n,1 builtins) or set. Like its command-line equiva-
               lent, you can use numeric or symbolic values, eg "$^D = 10" or
               "$^D = "st"".

       $SYSTEM_FD_MAX
       $^F     The maximum system file(1,n) descriptor, ordinarily 2.  System file(1,n)
               descriptors are passed to exec(3,n,1 builtins)()ed processes, while higher file(1,n)
               descriptors are not.  Also, during an open(2,3,n)(), system file(1,n)
               descriptors are preserved even if(3,n) the open(2,3,n)() fails.  (Ordinary
               file(1,n) descriptors are closed before the open(2,3,n)() is attempted.)
               The close-on-exec status of a file(1,n) descriptor will be decided
               according to the value of $^F when the corresponding file(1,n),
               pipe(2,8), or socket(2,7,n) was opened, not the time(1,2,n) of the exec(3,n,1 builtins)().

       $^H     WARNING: This variable is strictly for internal use only.  Its
               availability, behavior, and contents are subject to change
               without notice.

               This variable contains compile-time hints for the Perl inter-
               preter.  At the end of compilation of a BLOCK the value of this
               variable is restored to the value when the interpreter started
               to compile the BLOCK.

               When perl begins to parse any block construct that provides a
               lexical scope (e.g., eval body, required file(1,n), subroutine body,
               loop body, or conditional block), the existing value of $^H is
               saved, but its value is left unchanged.  When the compilation
               of the block is completed, it regains the saved value.  Between
               the points where its value is saved and restored, code that
               executes within BEGIN blocks is free to change the value of
               $^H.

               This behavior provides the semantic of lexical scoping, and is
               used in(1,8), for instance, the "use strict" pragma.

               The contents should be an integer; different bits of it are
               used for different pragmatic flags.  Here's an example:

                   sub add_100 { $^H |= 0x100 }

                   sub foo {
                       BEGIN { add_100() }
                       bar->baz($boon);
                   }

               Consider what happens during execution of the BEGIN block.  At
               this point the BEGIN block has already been compiled, but the
               body of foo() is still being compiled.  The new value of $^H
               will therefore be visible only while the body of foo() is being
               compiled.

               Substitution of the above BEGIN block with:

                   BEGIN { require strict; strict->import('vars') }

               demonstrates how "use strict 'vars'" is implemented.  Here's a
               conditional version(1,3,5) of the same lexical pragma:

                   BEGIN { require strict; strict->import('vars') if(3,n) $condition }

       %^H     WARNING: This variable is strictly for internal use only.  Its
               availability, behavior, and contents are subject to change
               without notice.

               The %^H hash provides the same scoping semantic as $^H.  This
               makes it useful for implementation of lexically scoped pragmas.

       $INPLACE_EDIT
       $^I     The current value of the inplace-edit extension.  Use "undef"
               to disable inplace editing.  (Mnemonic: value of -i switch.)

       $^M     By default, running out of memory is an untrappable, fatal
               error.  However, if(3,n) suitably built, Perl can use the contents
               of $^M as an emergency memory pool after die()ing.  Suppose
               that your Perl were compiled with -DPERL_EMERGENCY_SBRK and
               used Perl's malloc.  Then

                   $^M = 'a' x (1 << 16);

               would allocate a 64K buffer for use in(1,8) an emergency.  See the
               INSTALL file(1,n) in(1,8) the Perl distribution for information on how to
               enable this option.  To discourage casual use of this advanced
               feature, there is no English long name for this variable.

       $OSNAME
       $^O     The name of the operating system under which this copy of Perl
               was built, as determined during the configuration process.  The
               value is identical to $Config{'osname'}.  See also Config and
               the -V command-line switch(1,n) documented in(1,8) perlrun.

               In Windows platforms, $^O is not very helpful: since it is
               always "MSWin32", it doesn't tell the difference between
               95/98/ME/NT/2000/XP/CE/.NET.  Use Win32::GetOSName() or
               Win32::GetOSVersion() (see Win32 and perlport) to distinguish
               between the variants.

       ${^OPEN}
               An internal variable used by PerlIO.  A string(3,n) in(1,8) two parts,
               separated by a "\0" byte, the first part describes the input
               layers, the second part describes the output layers.

       $PERLDB
       $^P     The internal variable for debugging support.  The meanings of
               the various bits are subject to change, but currently indicate:

               0x01  Debug subroutine enter/exit.

               0x02  Line-by-line debugging.

               0x04  Switch off optimizations.

               0x08  Preserve more data for future interactive inspections.

               0x10  Keep info(1,5,n) about source lines on which a subroutine is
                     defined.

               0x20  Start with single-step on.

               0x40  Use subroutine address instead of name when reporting.

               0x80  Report "goto &subroutine" as well.

               0x100 Provide informative "file(1,n)" names for evals based on the
                     place they were compiled.

               0x200 Provide informative names to anonymous subroutines based
                     on the place they were compiled.

               0x400 Debug assertion subroutines enter/exit.

               Some bits may be relevant at compile-time only, some at run-
               time(1,2,n) only.  This is a new mechanism and the details may change.

       $LAST_REGEXP_CODE_RESULT
       $^R     The result of evaluation of the last successful "(?{ code })"
               regular expression assertion (see perlre).  May be written to.

       $EXCEPTIONS_BEING_CAUGHT
       $^S     Current state of the interpreter.

                   $^S         State
                   ---------   -------------------
                   undef       Parsing module/eval
                   true (1)    Executing an eval
                   false (0)   Otherwise

               The first state may happen in(1,8) $SIG{__DIE__} and $SIG{__WARN__}
               handlers.

       $BASETIME
       $^T     The time(1,2,n) at which the program began running, in(1,8) seconds since
               the epoch (beginning of 1970).  The values returned by the -M,
               -A, and -C filetests are based on this value.

       ${^TAINT}
               Reflects if(3,n) taint mode is on or off.  1 for on (the program was
               run with -T), 0 for off, -1 when only taint warnings are
               enabled (i.e. with -t or -TU).

       ${^UNICODE}
               Reflects certain Unicode settings of Perl.  See perlrun docu-
               mentation for the "-C" switch(1,n) for more information about the
               possible values. This variable is set(7,n,1 builtins) during Perl startup and
               is thereafter read-only.

       $PERL_VERSION
       $^V     The revision, version(1,3,5), and subversion of the Perl interpreter,
               represented as a string(3,n) composed of characters with those ordi-
               nals.  Thus in(1,8) Perl v5.6.0 it equals "chr(5) . chr(6) . chr(0)"
               and will return true for "$^V eq v5.6.0".  Note that the char-
               acters in(1,8) this string(3,n) value can potentially be in(1,8) Unicode
               range.

               This can be used to determine whether the Perl interpreter exe-
               cuting a script is in(1,8) the right range of versions.  (Mnemonic:
               use ^V for Version Control.)  Example:

                   warn "No \"our\" declarations!\n" if(3,n) $^V and $^V lt v5.6.0;

               To convert $^V into its string(3,n) representation use sprintf()'s
               "%vd" conversion:

                   printf(1,3,1 builtins) "version(1,3,5) is v%vd\n", $^V;  # Perl's version(1,3,5)

               See the documentation of "use VERSION" and "require VERSION"
               for a convenient way to fail if(3,n) the running Perl interpreter is
               too old.

               See also $] for an older representation of the Perl version.

       $WARNING
       $^W     The current value of the warning switch(1,n), initially true if(3,n) -w
               was used, false otherwise, but directly modifiable.  (Mnemonic:
               related to the -w switch.)  See also warnings.

       ${^WARNING_BITS}
               The current set(7,n,1 builtins) of warning checks enabled by the "use warnings"
               pragma.  See the documentation of "warnings" for more details.

       $EXECUTABLE_NAME
       $^X     The name used to execute the current copy of Perl, from C's
               "argv[0]".

               Depending on the host(1,5) operating system, the value of $^X may be
               a relative or absolute pathname of the perl program file(1,n), or
               may be the string(3,n) used to invoke perl but not the pathname of
               the perl program file.  Also, most operating systems permit
               invoking programs that are not in(1,8) the PATH environment vari-
               able, so there is no guarantee that the value of $^X is in(1,8)
               PATH.  For VMS, the value may or may not include a version(1,3,5) num-
               ber.

               You usually can use the value of $^X to re-invoke an indepen-
               dent copy of the same perl that is currently running, e.g.,

                 @first_run = `$^X -le "print int rand(1,3) 100 for 1..100"`;

               But recall that not all operating systems support forking or
               capturing of the output of commands, so this complex statement
               may not be portable.

               It is not safe to use the value of $^X as a path name of a
               file(1,n), as some operating systems that have a mandatory suffix on
               executable files do not require use of the suffix when invoking
               a command.  To convert the value of $^X to a path name, use the
               following statements:

               # Build up a set(7,n,1 builtins) of file(1,n) names (not command names).
                 use Config;
                 $this_perl = $^X;
                 if(3,n) ($^O ne 'VMS')
                    {$this_perl .= $Config{_exe}
                         unless $this_perl =~ m/$Config{_exe}$/i;}

               Because many operating systems permit anyone with read(2,n,1 builtins) access(2,5)
               to the Perl program file(1,n) to make a copy of it, patch the copy,
               and then execute the copy, the security-conscious Perl program-
               mer should take care to invoke the installed copy of perl, not
               the copy referenced by $^X.  The following statements accom-
               plish this goal, and produce a pathname that can be invoked as
               a command or referenced as a file.

                 use Config;
                 $secure_perl_path = $Config{perlpath};
                 if(3,n) ($^O ne 'VMS')
                    {$secure_perl_path .= $Config{_exe}
                         unless $secure_perl_path =~ m/$Config{_exe}$/i;}

       ARGV    The special filehandle that iterates over command-line file-
               names in(1,8) @ARGV. Usually written as the null filehandle in(1,8) the
               angle operator "<>". Note that currently "ARGV" only has its
               magical effect within the "<>" operator; elsewhere it is just a
               plain filehandle corresponding to the last file(1,n) opened by "<>".
               In particular, passing "\*ARGV" as a parameter to a function
               that expects a filehandle may not cause your function to auto-
               matically read(2,n,1 builtins) the contents of all the files in(1,8) @ARGV.

       $ARGV   contains the name of the current file(1,n) when reading from <>.

       @ARGV   The array @ARGV contains the command-line arguments intended
               for the script.  $#ARGV is generally the number of arguments
               minus one, because $ARGV[0] is the first argument, not the pro-
               gram's command name itself.  See $0 for the command name.

       ARGVOUT The special filehandle that points to the currently open(2,3,n) output
               file(1,n) when doing edit-in-place processing with -i.  Useful when
               you have to do a lot of inserting and don't want to keep modi-
               fying $_.  See perlrun for the -i switch.

       @F      The array @F contains the fields of each line read(2,n,1 builtins) in(1,8) when
               autosplit mode is turned on.  See perlrun for the -a switch.
               This array is package-specific, and must be declared or given a
               full package name if(3,n) not in(1,8) package main when running under
               "strict 'vars'".

       @INC    The array @INC contains the list of places that the "do EXPR",
               "require", or "use" constructs look(1,8,3 Search::Dict) for their library files.
               It initially consists of the arguments to any -I command-line
               switches, followed by the default Perl library, probably
               /usr/local/lib/perl, followed by ".", to represent the current
               directory.  ("." will not be appended if(3,n) taint checks are
               enabled, either by "-T" or by "-t".)  If you need to modify
               this at runtime, you should use the "use lib" pragma to get the
               machine-dependent library properly loaded also:

                   use lib '/mypath/libdir/';
                   use SomeMod;

               You can also insert hooks into the file(1,n) inclusion system by
               putting Perl code directly into @INC.  Those hooks may be sub-
               routine references, array references or blessed objects.  See
               "require" in(1,8) perlfunc for details.

       @_      Within a subroutine the array @_ contains the parameters passed
               to that subroutine.  See perlsub.

       %INC    The hash %INC contains entries for each filename included via
               the "do", "require", or "use" operators.  The key is the file-
               name you specified (with module names converted to pathnames),
               and the value is the location of the file(1,n) found.  The "require"
               operator uses this hash to determine whether a particular file(1,n)
               has already been included.

               If the file(1,n) was loaded via a hook (e.g. a subroutine reference,
               see "require" in(1,8) perlfunc for a description of these hooks),
               this hook is by default inserted into %INC in(1,8) place of a file-
               name.  Note, however, that the hook may have set(7,n,1 builtins) the %INC entry
               by itself to provide some more specific info.

       %ENV
       $ENV{expr(1,3,n)}
               The hash %ENV contains your current environment.  Setting a
               value in(1,8) "ENV" changes the environment for any child processes
               you subsequently fork() off.

       %SIG
       $SIG{expr(1,3,n)}
               The hash %SIG contains signal(2,7) handlers for signals.  For exam-
               ple:

                   sub handler {       # 1st argument is signal(2,7) name
                       my($sig) = @_;
                       print "Caught a SIG$sig--shutting down\n";
                       close(2,7,n)(LOG);
                       exit(3,n,1 builtins)(0);
                   }

                   $SIG{'INT'}  = \&handler;
                   $SIG{'QUIT'} = \&handler;
                   ...
                   $SIG{'INT'}  = 'DEFAULT';   # restore default action
                   $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT

               Using a value of 'IGNORE' usually has the effect of ignoring
               the signal(2,7), except for the "CHLD" signal.  See perlipc for more
               about this special case.

               Here are some other examples:

                   $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not recommended)
                   $SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
                   $SIG{"PIPE"} = *Plumber;    # somewhat esoteric
                   $SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??

               Be sure not to use a bareword as the name of a signal(2,7) handler,
               lest you inadvertently call it.

               If your system has the sigaction() function then signal(2,7) han-
               dlers are installed using it.  This means you get reliable sig-
               nal(2,7) handling.

               The default delivery policy of signals changed in(1,8) Perl 5.8.0
               from immediate (also known as "unsafe") to deferred, also known
               as "safe signals".  See perlipc for more information.

               Certain internal hooks can be also set(7,n,1 builtins) using the %SIG hash.
               The routine indicated by $SIG{__WARN__} is called when a warn-
               ing message is about to be printed.  The warning message is
               passed as the first argument.  The presence of a __WARN__ hook
               causes the ordinary printing of warnings to STDERR to be sup-
               pressed.  You can use this to save warnings in(1,8) a variable, or
               turn warnings into fatal errors, like this:

                   local $SIG{__WARN__} = sub { die $_[0] };
                   eval $proggie;

               The routine indicated by $SIG{__DIE__} is called when a fatal
               exception is about to be thrown.  The error(8,n) message is passed
               as the first argument.  When a __DIE__ hook routine returns,
               the exception processing continues as it would have in(1,8) the
               absence of the hook, unless the hook routine itself exits via a
               "goto", a loop exit(3,n,1 builtins), or a die().  The "__DIE__" handler is
               explicitly disabled during the call, so that you can die from a
               "__DIE__" handler.  Similarly for "__WARN__".

               Due to an implementation glitch, the $SIG{__DIE__} hook is
               called even inside an eval().  Do not use this to rewrite a
               pending exception in(1,8) $@, or as a bizarre substitute for over-
               riding CORE::GLOBAL::die().  This strange action at a distance
               may be fixed in(1,8) a future release so that $SIG{__DIE__} is only
               called if(3,n) your program is about to exit(3,n,1 builtins), as was the original
               intent.  Any other use is deprecated.

               "__DIE__"/"__WARN__" handlers are very special in(1,8) one respect:
               they may be called to report (probable) errors found by the
               parser.  In such a case the parser may be in(1,8) inconsistent
               state, so any attempt to evaluate Perl code from such a handler
               will probably result in(1,8) a segfault.  This means that warnings
               or errors that result from parsing Perl should be used with
               extreme caution, like this:

                   require Carp if(3,n) defined $^S;
                   Carp::confess("Something wrong") if(3,n) defined &Carp::confess;
                   die "Something wrong, but could not load(7,n) Carp to give backtrace...
                        To see backtrace try starting Perl with -MCarp switch(1,n)";

               Here the first line will load(7,n) Carp unless it is the parser who
               called the handler.  The second line will print backtrace and
               die if(3,n) Carp was available.  The third line will be executed
               only if(3,n) Carp was not available.

               See "die" in(1,8) perlfunc, "warn" in(1,8) perlfunc, "eval" in(1,8) perlfunc,
               and warnings for additional information.

       Error Indicators

       The variables $@, $!, $^E, and $? contain information about different
       types of error(8,n) conditions that may appear during execution of a Perl
       program.  The variables are shown ordered by the "distance" between the
       subsystem which reported the error(8,n) and the Perl process.  They corre-
       spond to errors detected by the Perl interpreter, C library, operating
       system, or an external program, respectively.

       To illustrate the differences between these variables, consider the
       following Perl expression, which uses a single-quoted string:

           eval q{
               open(2,3,n) my $pipe(2,8), "/cdrom/install |" or die $!;
               my @res = <$pipe(2,8)>;
               close(2,7,n) $pipe(2,8) or die "bad pipe: $?, $!";
           };

       After execution of this statement all 4 variables may have been set.

       $@ is set(7,n,1 builtins) if(3,n) the string(3,n) to be "eval"-ed did not compile (this may hap-
       pen if(3,n) "open(2,3,n)" or "close(2,7,n)" were imported with bad prototypes), or if(3,n) Perl
       code executed during evaluation die()d .  In these cases the value of
       $@ is the compile error(8,n), or the argument to "die" (which will interpo-
       late $! and $?!).  (See also Fatal, though.)

       When the eval() expression above is executed, open(2,3,n)(), "<PIPE>", and
       "close(2,7,n)" are translated to calls in(1,8) the C run-time library and thence to
       the operating system kernel.  $! is set(7,n,1 builtins) to the C library's "errno" if(3,n)
       one of these calls fails.

       Under a few operating systems, $^E may contain a more verbose error(8,n)
       indicator, such as in(1,8) this case, "CDROM tray not closed."  Systems that
       do not support extended error(8,n) messages leave $^E the same as $!.

       Finally, $? may be set(7,n,1 builtins) to non-0 value if(3,n) the external program
       /cdrom/install fails.  The upper eight bits reflect specific error(8,n) con-
       ditions encountered by the program (the program's exit(3,n,1 builtins)() value).   The
       lower eight bits reflect mode of failure, like signal(2,7) death and core
       dump information  See wait(2) for details.  In contrast to $! and $^E,
       which are set(7,n,1 builtins) only if(3,n) error(8,n) condition is detected, the variable $? is
       set(7,n,1 builtins) on each "wait" or pipe(2,8) "close(2,7,n)", overwriting the old value.  This is
       more like $@, which on every eval() is always set(7,n,1 builtins) on failure and
       cleared on success.

       For more details, see the individual descriptions at $@, $!, $^E, and
       $?.

       Technical Note on the Syntax of Variable Names

       Variable names in(1,8) Perl can have several formats.  Usually, they must
       begin with a letter or underscore, in(1,8) which case they can be arbitrar-
       ily long (up to an internal limit of 251 characters) and may contain
       letters, digits, underscores, or the special sequence "::" or "'".  In
       this case, the part before the last "::" or "'" is taken to be a pack-
       age qualifier; see perlmod.

       Perl variable names may also be a sequence of digits or a single punc-
       tuation or control character.  These names are all reserved for special
       uses by Perl; for example, the all-digits names are used to hold data
       captured by backreferences after a regular expression match.  Perl has
       a special syntax for the single-control-character names: It understands
       "^X" (caret "X") to mean the control-"X" character.  For example, the
       notation $^W (dollar-sign caret "W") is the scalar variable whose name
       is the single character control-"W".  This is better than typing a lit-
       eral control-"W" into your program.

       Finally, new in(1,8) Perl 5.6, Perl variable names may be alphanumeric
       strings that begin with control characters (or better yet, a caret).
       These variables must be written in(1,8) the form "${^Foo}"; the braces are
       not optional.  "${^Foo}" denotes the scalar variable whose name is a
       control-"F" followed by two "o"'s.  These variables are reserved for
       future special uses by Perl, except for the ones that begin with "^_"
       (control-underscore or caret-underscore).  No control-character name
       that begins with "^_" will acquire a special meaning in(1,8) any future ver-
       sion(1,3,5) of Perl; such names may therefore be used safely in(1,8) programs.  $^_
       itself, however, is reserved.

       Perl identifiers that begin with digits, control characters, or punctu-
       ation characters are exempt from the effects of the "package" declara-
       tion and are always forced to be in(1,8) package "main"; they are also
       exempt from "strict 'vars'" errors.  A few other names are also exempt
       in(1,8) these ways:

               ENV             STDIN
               INC             STDOUT
               ARGV            STDERR
               ARGVOUT         _
               SIG

       In particular, the new special "${^_XYZ}" variables are always taken to
       be in(1,8) package "main", regardless of any "package" declarations
       presently in(1,8) scope.

BUGS
       Due to an unfortunate accident of Perl's implementation, "use English"
       imposes a considerable performance penalty on all regular expression
       matches in(1,8) a program, regardless of whether they occur in(1,8) the scope of
       "use English".  For that reason, saying "use English" in(1,8) libraries is
       strongly discouraged.  See the Devel::SawAmpersand module documentation
       from CPAN ( http://www.cpan.org/modules/by-module/Devel/ ) for more
       information.

       Having to even think about the $^S variable in(1,8) your exception handlers
       is simply wrong.  $SIG{__DIE__} as currently implemented invites griev-
       ous and difficult to track down errors.  Avoid it and use an "END{}" or
       CORE::GLOBAL::die override instead.



perl v5.8.5                       2004-04-23                        PERLVAR(1)

References for this manual (incoming links)