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perlsub(1) - perlsub - Perl subroutines - man 1 perlsub

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PERLSUB(1)             Perl Programmers Reference Guide             PERLSUB(1)

       perlsub - Perl subroutines

       To declare subroutines:

           sub NAME;                     # A "forward" declaration.
           sub NAME(PROTO);              #  ditto, but with prototypes
           sub NAME : ATTRS;             #  with attributes
           sub NAME(PROTO) : ATTRS;      #  with attributes and prototypes

           sub NAME BLOCK                # A declaration and a definition.
           sub NAME(PROTO) BLOCK         #  ditto, but with prototypes
           sub NAME : ATTRS BLOCK        #  with attributes
           sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes

       To define an anonymous subroutine at runtime:

           $subref = sub BLOCK;                 # no proto
           $subref = sub (PROTO) BLOCK;         # with proto
           $subref = sub : ATTRS BLOCK;         # with attributes
           $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

       To import subroutines:

           use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

           NAME(LIST);    # & is optional with parentheses.
           NAME LIST;     # Parentheses optional if(3,n) predeclared/imported.
           &NAME(LIST);   # Circumvent prototypes.
           &NAME;         # Makes current @_ visible to called subroutine.

       Like many languages, Perl provides for user-defined subroutines.  These
       may be located anywhere in(1,8) the main program, loaded in(1,8) from other files
       via the "do", "require", or "use" keywords, or generated on the fly
       using "eval" or anonymous subroutines.  You can even call a function
       indirectly using a variable containing its name or a CODE reference.

       The Perl model for function call and return values is simple: all func-
       tions are passed as parameters one single flat list of scalars, and all
       functions likewise return to their caller one single flat list of
       scalars.  Any arrays or hashes in(1,8) these call and return lists will col-
       lapse, losing their identities--but you may always use pass-by-refer-
       ence instead to avoid this.  Both call and return lists may contain as
       many or as few scalar elements as you'd like.  (Often a function with-
       out an explicit return statement is called a subroutine, but there's
       really no difference from Perl's perspective.)

       Any arguments passed in(1,8) show up in(1,8) the array @_.  Therefore, if(3,n) you
       called a function with two arguments, those would be stored in(1,8) $_[0]
       and $_[1].  The array @_ is a local array, but its elements are aliases
       for the actual scalar parameters.  In particular, if(3,n) an element $_[0]
       is updated, the corresponding argument is updated (or an error(8,n) occurs
       if(3,n) it is not updatable).  If an argument is an array or hash element
       which did not exist when the function was called, that element is cre-
       ated only when (and if(3,n)) it is modified or a reference to it is taken.
       (Some earlier versions of Perl created the element whether or not the
       element was assigned to.)  Assigning to the whole array @_ removes that
       aliasing, and does not update(7,n) any arguments.

       The return value of a subroutine is the value of the last expression
       evaluated by that sub, or the empty list in(1,8) the case of an empty sub.
       More explicitly, a "return" statement may be used to exit(3,n,1 builtins) the subrou-
       tine, optionally specifying the returned value, which will be evaluated
       in(1,8) the appropriate context (list, scalar, or void) depending on the
       context of the subroutine call.  If you specify no return value, the
       subroutine returns an empty list in(1,8) list context, the undefined value
       in(1,8) scalar context, or nothing in(1,8) void context.  If you return one or
       more aggregates (arrays and hashes), these will be flattened together
       into one large indistinguishable list.

       Perl does not have named(5,8) formal parameters.  In practice all you do is
       assign to a "my()" list of these.  Variables that aren't declared to be
       private are global variables.  For gory details on creating private
       variables, see "Private Variables via my()" and "Temporary Values via
       local()".  To create protected environments for a set(7,n,1 builtins) of functions in(1,8) a
       separate package (and probably a separate file(1,n)), see "Packages" in(1,8)


           sub max {
               my $max = shift(@_);
               foreach $foo (@_) {
                   $max = $foo if(3,n) $max < $foo;
               return $max;
           $bestday = max($mon,$tue,$wed,$thu,$fri);


           # get a line, combining continuation lines
           #  that start with whitespace

           sub get_line {
               $thisline = $lookahead;  # global variables!
               LINE: while (defined($lookahead = <STDIN>)) {
                   if(3,n) ($lookahead =~ /^[ \t]/) {
                       $thisline .= $lookahead;
                   else {
                       last LINE;
               return $thisline;

           $lookahead = <STDIN>;       # get first line
           while (defined($line = get_line())) {

       Assigning to a list of private variables to name your arguments:

           sub maybeset {
               my($key, $value) = @_;
               $Foo{$key} = $value unless $Foo{$key};

       Because the assignment copies the values, this also has the effect of
       turning call-by-reference into call-by-value.  Otherwise a function is
       free to do in-place modifications of @_ and change its caller's values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }

       You aren't allowed to modify constants in(1,8) this way, of course.  If an
       argument were actually literal and you tried to change it, you'd take a
       (presumably fatal) exception.   For example, this won't work:


       It would be much safer if(3,n) the "upcase_in()" function were written to
       return a copy of its parameters instead of changing them in(1,8) place:

           ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
           sub upcase {
               return unless defined wantarray;  # void context, do nothing
               my @parms = @_;
               for (@parms) { tr/a-z/A-Z/ }
               return wantarray ? @parms : $parms[0];

       Notice how this (unprototyped) function doesn't care whether it was
       passed real scalars or arrays.  Perl sees all arguments as one big,
       long, flat parameter list in(1,8) @_.  This is one area where Perl's simple
       argument-passing style shines.  The "upcase()" function would work per-
       fectly well without changing the "upcase()" definition even if(3,n) we fed
       it things like this:

           @newlist   = upcase(@list1, @list2);
           @newlist   = upcase( split(1,n) /:/, $var );

       Do not, however, be tempted to do this:

           (@a, @b)   = upcase(@list1, @list2);

       Like the flattened incoming parameter list, the return list is also
       flattened on return.  So all you have managed to do here is stored
       everything in(1,8) @a and made @b empty.  See "Pass by Reference" for alter-

       A subroutine may be called using an explicit "&" prefix.  The "&" is
       optional in(1,8) modern Perl, as are parentheses if(3,n) the subroutine has been
       predeclared.  The "&" is not optional when just naming the subroutine,
       such as when it's used as an argument to defined() or undef().  Nor is
       it optional when you want to do an indirect subroutine call with a sub-
       routine name or reference using the "&$subref()" or "&{$subref}()" con-
       structs, although the "$subref->()" notation solves that problem.  See
       perlref for more about all that.

       Subroutines may be called recursively.  If a subroutine is called using
       the "&" form, the argument list is optional, and if(3,n) omitted, no @_
       array is set(7,n,1 builtins) up for the subroutine: the @_ array at the time(1,2,n) of the
       call is visible to subroutine instead.  This is an efficiency mechanism
       that new users(1,5) may wish to avoid.

           &foo(1,2,3);        # pass three arguments
           foo(1,2,3);         # the same

           foo();              # pass a null list
           &foo();             # the same

           &foo;               # foo() get current args, like foo(@_) !!
           foo;                # like foo() IFF sub foo predeclared, else "foo"

       Not only does the "&" form make the argument list optional, it also
       disables any prototype checking on arguments you do provide.  This is
       partly for historical reasons, and partly for having a convenient way
       to cheat if(3,n) you know what you're doing.  See Prototypes below.

       Subroutines whose names are in(1,8) all upper case are reserved to the Perl
       core, as are modules whose names are in(1,8) all lower case.  A subroutine
       in(1,8) all capitals is a loosely-held convention meaning it will be called
       indirectly by the run-time system itself, usually due to a triggered
       event.  Subroutines that do special, pre-defined things include
       "AUTOLOAD", "CLONE", "DESTROY" plus all functions mentioned in(1,8) perltie
       and PerlIO::via.

       The "BEGIN", "CHECK", "INIT" and "END" subroutines are not so much sub-
       routines as named(5,8) special code blocks, of which you can have more than
       one in(1,8) a package, and which you can not call explicitely.  See "BEGIN,
       CHECK, INIT and END" in(1,8) perlmod

       Private Variables via my()


           my $foo;            # declare $foo lexically local
           my (@wid, %get);    # declare list of variables local
           my $foo = "flurp";  # declare $foo lexical, and init it
           my @oof = @bar;     # declare @oof lexical, and init it
           my $x : Foo = $y;   # similar, with an attribute applied

       WARNING: The use of attribute lists on "my" declarations is still
       evolving.  The current semantics and interface are subject to change.
       See attributes and Attribute::Handlers.

       The "my" operator declares the listed variables to be lexically con-
       fined to the enclosing block, conditional ("if(3,n)/unless/elsif/else"),
       loop ("for/foreach/while/until/continue"), subroutine, "eval", or
       "do/require/use"'d file.  If more than one value is listed, the list
       must be placed in(1,8) parentheses.  All listed elements must be legal lval-
       ues.  Only alphanumeric identifiers may be lexically scoped--magical
       built-ins like $/ must currently be "local"ized with "local" instead.

       Unlike dynamic variables created by the "local" operator, lexical vari-
       ables declared with "my" are totally hidden from the outside world,
       including any called subroutines.  This is true if(3,n) it's the same sub-
       routine called from itself or elsewhere--every call gets(3,n) its own copy.

       This doesn't mean that a "my" variable declared in(1,8) a statically enclos-
       ing lexical scope would be invisible.  Only dynamic scopes are cut off.
       For example, the "bumpx()" function below has access(2,5) to the lexical $x
       variable because both the "my" and the "sub" occurred at the same
       scope, presumably file(1,n) scope.

           my $x = 10;
           sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the scope it is
       being evaluated in(1,8), so long as the names aren't hidden by declarations
       within the "eval()" itself.  See perlref.

       The parameter list to my() may be assigned to if(3,n) desired, which allows
       you to initialize your variables.  (If no initializer is given for a
       particular variable, it is created with the undefined value.)  Commonly
       this is used to name input parameters to a subroutine.  Examples:

           $arg = "fred";        # "global" variable
           $n = cube_root(27);
           print "$arg thinks the root is $n\n";
        fred thinks the root is 3

           sub cube_root {
               my $arg = shift;  # name doesn't matter
               $arg **= 1/3;
               return $arg;

       The "my" is simply a modifier on something you might assign to.  So
       when you do assign to variables in(1,8) its argument list, "my" doesn't
       change whether those variables are viewed as a scalar or an array.  So

           my ($foo) = <STDIN>;                # WRONG?
           my @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           my $foo = <STDIN>;

       supplies a scalar context.  But the following declares only one vari-

           my $foo, $bar = 1;                  # WRONG

       That has the same effect as

           my $foo;
           $bar = 1;

       The declared variable is not introduced (is not visible) until after
       the current statement.  Thus,

           my $x = $x;

       can be used to initialize a new $x with the value of the old $x, and
       the expression

           my $x = 123 and $x == 123

       is false unless the old $x happened to have the value 123.

       Lexical scopes of control structures are not bounded precisely by the
       braces that delimit their controlled blocks; control expressions are
       part of that scope, too.  Thus in(1,8) the loop

           while (my $line = <>) {
               $line = lc $line;
           } continue {
               print $line;

       the scope of $line extends from its declaration throughout the rest of
       the loop construct (including the "continue" clause), but not beyond
       it.  Similarly, in(1,8) the conditional

           if(3,n) ((my $answer = <STDIN>) =~ /^yes$/i) {
           } elsif ($answer =~ /^no$/i) {
           } else {
               chomp $answer;
               die "'$answer' is neither 'yes' nor 'no'";

       the scope of $answer extends from its declaration through the rest of
       that conditional, including any "elsif" and "else" clauses, but not
       beyond it.  See "Simple statements" in(1,8) perlsyn for information on the
       scope of variables in(1,8) statements with modifiers.

       The "foreach" loop defaults to scoping its index variable dynamically
       in(1,8) the manner of "local".  However, if(3,n) the index variable is prefixed
       with the keyword "my", or if(3,n) there is already a lexical by that name in(1,8)
       scope, then a new lexical is created instead.  Thus in(1,8) the loop

           for my $i (1, 2, 3) {

       the scope of $i extends to the end of the loop, but not beyond it, ren-
       dering the value of $i inaccessible within "some_function()".

       Some users(1,5) may wish to encourage the use of lexically scoped variables.
       As an aid to catching implicit uses to package variables, which are
       always global, if(3,n) you say

           use strict 'vars';

       then any variable mentioned from there to the end of the enclosing
       block must either refer to a lexical variable, be predeclared via "our"
       or "use vars", or else must be fully qualified with the package name.
       A compilation error(8,n) results otherwise.  An inner block may countermand
       this with "no strict 'vars'".

       A "my" has both a compile-time and a run-time effect.  At compile time(1,2,n),
       the compiler takes notice of it.  The principal usefulness of this is
       to quiet "use strict 'vars'", but it is also essential for generation
       of closures as detailed in(1,8) perlref.  Actual initialization is delayed
       until run time(1,2,n), though, so it gets(3,n) executed at the appropriate time(1,2,n),
       such as each time(1,2,n) through a loop, for example.

       Variables declared with "my" are not part of any package and are there-
       fore never fully qualified with the package name.  In particular,
       you're not allowed to try to make a package variable (or other global)

           my $pack::var;      # ERROR!  Illegal syntax
           my $_;              # also illegal (currently)

       In fact, a dynamic variable (also known as package or global variables)
       are still accessible using the fully qualified "::" notation even while
       a lexical of the same name is also visible:

           package main;
           local $x = 10;
           my    $x = 20;
           print "$x and $::x\n";

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of a file(1,n) to hide
       any such identifiers from the world outside that file.  This is similar
       in(1,8) spirit to C's static variables when they are used at the file(1,n) level.
       To do this with a subroutine requires the use of a closure (an anony-
       mous function that accesses enclosing lexicals).  If you want to create
       a private subroutine that cannot be called from outside that block, it
       can declare a lexical variable containing an anonymous sub reference:

           my $secret_version = '1.001-beta';
           my $secret_sub = sub { print $secret_version };

       As long as the reference is never returned by any function within the
       module, no outside module can see the subroutine, because its name is
       not in(1,8) any package's symbol table.  Remember that it's not REALLY
       called $some_pack::secret_version or anything; it's just $secret_ver-
       sion, unqualified and unqualifiable.

       This does not work with object methods, however; all object methods
       have to be in(1,8) the symbol table of some package to be found.  See "Func-
       tion Templates" in(1,8) perlref for something of a work-around to this.

       Persistent Private Variables

       Just because a lexical variable is lexically (also called statically)
       scoped to its enclosing block, "eval", or "do" FILE, this doesn't mean
       that within a function it works like a C static.  It normally works
       more like a C auto(5,8), but with implicit garbage collection.

       Unlike local variables in(1,8) C or C++, Perl's lexical variables don't nec-
       essarily get recycled just because their scope has exited.  If some-
       thing more permanent is still aware of the lexical, it will stick
       around.  So long as something else references a lexical, that lexical
       won't be freed--which is as it should be.  You wouldn't want memory
       being free until you were done using it, or kept around once you were
       done.  Automatic garbage collection takes care of this for you.

       This means that you can pass back or save away references to lexical
       variables, whereas to return a pointer to a C auto(5,8) is a grave error.
       It also gives us a way to simulate C's function statics.  Here's a
       mechanism for giving a function private variables with both lexical
       scoping and a static lifetime.  If you do want to create something like
       C's static variables, just enclose the whole function in(1,8) an extra
       block, and put the static variable outside the function but in(1,8) the

               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
           # $secret_val now becomes unreachable by the outside
           # world, but retains its value between calls to gimme_another

       If this function is being sourced in(1,8) from a separate file(1,n) via "require"
       or "use", then this is probably just fine.  If it's all in(1,8) the main
       program, you'll need to arrange for the "my" to be executed early,
       either by putting the whole block above your main program, or more
       likely, placing merely a "BEGIN" code block around it to make sure it
       gets(3,n) executed before your program starts to run:

           BEGIN {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;

       See "BEGIN, CHECK, INIT and END" in(1,8) perlmod about the special triggered
       code blocks, "BEGIN", "CHECK", "INIT" and "END".

       If declared at the outermost scope (the file(1,n) scope), then lexicals work
       somewhat like C's file(1,n) statics.  They are available to all functions in(1,8)
       that same file(1,n) declared below them, but are inaccessible from outside
       that file.  This strategy is sometimes used in(1,8) modules to create pri-
       vate variables that the whole module can see.

       Temporary Values via local()

       WARNING: In general, you should be using "my" instead of "local",
       because it's faster and safer.  Exceptions to this include the global
       punctuation variables, global filehandles and formats, and direct
       manipulation of the Perl symbol table itself.  "local" is mostly used
       when the current value of a variable must be visible to called subrou-


           # localization of values

           local $foo;                 # make $foo dynamically local
           local (@wid, %get);         # make list of variables local
           local $foo = "flurp";       # make $foo dynamic, and init it
           local @oof = @bar;          # make @oof dynamic, and init it

           local $hash{key} = "val";   # sets a local value for this hash entry
           local ($cond ? $v1 : $v2);  # several types of lvalues support
                                       # localization

           # localization of symbols

           local *FH;                  # localize $FH, @FH, %FH, &FH  ...
           local *merlyn = *randal;    # now $merlyn is really $randal, plus
                                       #     @merlyn is really @randal, etc
           local *merlyn = 'randal';   # SAME THING: promote 'randal' to *randal
           local *merlyn = \$randal;   # just alias $merlyn, not @merlyn etc

       A "local" modifies its listed variables to be "local" to the enclosing
       block, "eval", or "do FILE"--and to any subroutine called from within
       that block.  A "local" just gives temporary values to global (meaning
       package) variables.  It does not create a local variable.  This is
       known as dynamic scoping.  Lexical scoping is done with "my", which
       works more like C's auto(5,8) declarations.

       Some types of lvalues can be localized as well : hash and array ele-
       ments and slices, conditionals (provided that their result is always
       localizable), and symbolic references.  As for simple variables, this
       creates new, dynamically scoped values.

       If more than one variable or expression is given to "local", they must
       be placed in(1,8) parentheses.  This operator works by saving the current
       values of those variables in(1,8) its argument list on a hidden stack and
       restoring them upon exiting the block, subroutine, or eval.  This means
       that called subroutines can also reference the local variable, but not
       the global one.  The argument list may be assigned to if(3,n) desired, which
       allows you to initialize your local variables.  (If no initializer is
       given for a particular variable, it is created with an undefined

       Because "local" is a run-time operator, it gets(3,n) executed each time(1,2,n)
       through a loop.  Consequently, it's more efficient to localize your
       variables outside the loop.

       Grammatical note on local()

       A "local" is simply a modifier on an lvalue expression.  When you
       assign to a "local"ized variable, the "local" doesn't change whether
       its list is viewed as a scalar or an array.  So

           local($foo) = <STDIN>;
           local @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           local $foo = <STDIN>;

       supplies a scalar context.

       Localization of special variables

       If you localize a special variable, you'll be giving a new value to it,
       but its magic(4,5) won't go away.  That means that all side-effects related
       to this magic(4,5) still work with the localized value.

       This feature allows code like this to work :

           # Read the whole contents of FILE in(1,8) $slurp
           { local $/ = undef; $slurp = <FILE>; }

       Note, however, that this restricts localization of some values ; for
       example, the following statement dies, as of perl 5.9.0, with an error(8,n)
       Modification of a read-only value attempted, because the $1 variable is
       magical and read-only :

           local $1 = 2;

       Similarly, but in(1,8) a way more difficult to spot, the following snippet
       will die in(1,8) perl 5.9.0 :

           sub f { local $_ = "foo"; print }
           for ($1) {
               # now $_ is aliased to $1, thus is magic(4,5) and readonly

       See next section for an alternative to this situation.

       WARNING: Localization of tied arrays and hashes does not currently work
       as described.  This will be fixed in(1,8) a future release of Perl; in(1,8) the
       meantime, avoid code that relies on any particular behaviour of local-
       ising tied arrays or hashes (localising individual elements is still
       okay).  See "Localising Tied Arrays and Hashes Is Broken" in(1,8)
       perl58delta for more details.

       Localization of globs

       The construct

           local *name;

       creates a whole new symbol table entry for the glob(1,3,7,n) "name" in(1,8) the cur-
       rent package.  That means that all variables in(1,8) its glob(1,3,7,n) slot ($name,
       @name, %name, &name, and the "name" filehandle) are dynamically reset.

       This implies, among other things, that any magic(4,5) eventually carried by
       those variables is locally lost.  In other words, saying "local */"
       will not have any effect on the internal value of the input record sep-

       Notably, if(3,n) you want to work with a brand new value of the default
       scalar $_, and avoid the potential problem listed above about $_ previ-
       ously carrying a magic(4,5) value, you should use "local *_" instead of
       "local $_".

       Localization of elements of composite types

       It's also worth taking a moment to explain what happens when you
       "local"ize a member of a composite type (i.e. an array or hash ele-
       ment).  In this case, the element is "local"ized by name. This means
       that when the scope of the "local()" ends, the saved value will be
       restored to the hash element whose key was named(5,8) in(1,8) the "local()", or
       the array element whose index was named(5,8) in(1,8) the "local()".  If that ele-
       ment was deleted while the "local()" was in(1,8) effect (e.g. by a
       "delete()" from a hash or a "shift()" of an array), it will spring back
       into existence, possibly extending an array and filling in(1,8) the skipped
       elements with "undef".  For instance, if(3,n) you say

           %hash = ( 'This' => 'is', 'a' => 'test' );
           @ary  = ( 0..5 );
                local($ary[5]) = 6;
                local($hash{'a'}) = 'drill';
                while (my $e = pop(@ary)) {
                    print "$e . . .\n";
                    last unless $e > 3;
                if(3,n) (@ary) {
                    $hash{'only a'} = 'test';
                    delete $hash{'a'};
           print join(1,n)(' ', map { "$_ $hash{$_}" } sort(1,3) keys %hash),".\n";
           print "The array has ",scalar(@ary)," elements: ",
                 join(1,n)(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

           6 . . .
           4 . . .
           3 . . .
           This is a test only a test.
           The array has 6 elements: 0, 1, 2, undef, undef, 5

       The behavior of local() on non-existent members of composite types is
       subject to change in(1,8) future.

       Lvalue subroutines

       WARNING: Lvalue subroutines are still experimental and the implementa-
       tion may change in(1,8) future versions of Perl.

       It is possible to return a modifiable value from a subroutine.  To do
       this, you have to declare the subroutine to return an lvalue.

           my $val;
           sub canmod : lvalue {
               # return $val; this doesn't work, don't say "return"
           sub nomod {

           canmod() = 5;   # assigns to $val
           nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the right-hand side
       of assignment is determined as if(3,n) the subroutine call is replaced by a
       scalar. For example, consider:

           data(2,3) = get_data(3,4);

       Both subroutines here are called in(1,8) a scalar context, while in:

           (data(2,3)) = get_data(3,4);

       and in:

           (data(2),data(3)) = get_data(3,4);

       all the subroutines are called in(1,8) a list context.

       Lvalue subroutines are EXPERIMENTAL
           They appear to be convenient, but there are several reasons to be

           You can't use the return keyword, you must pass out the value
           before falling out of subroutine scope. (see comment in(1,8) example
           above).  This is usually not a problem, but it disallows an
           explicit return out of a deeply nested loop, which is sometimes a
           nice(1,2) way out.

           They violate encapsulation.  A normal mutator can check the sup-
           plied argument before setting the attribute it is protecting, an
           lvalue subroutine never gets(3,n) that chance.  Consider;

               my $some_array_ref = [];    # protected by mutators ??

               sub set_arr {               # normal mutator
                   my $val = shift;
                   die("expected array, you supplied ", ref $val)
                      unless ref $val eq 'ARRAY';
                   $some_array_ref = $val;
               sub set_arr_lv : lvalue {   # lvalue mutator

               # set_arr_lv cannot stop this !
               set_arr_lv() = { a => 1 };

       Passing Symbol Table Entries (typeglobs)

       WARNING: The mechanism described in(1,8) this section was originally the
       only way to simulate pass-by-reference in(1,8) older versions of Perl.
       While it still works fine in(1,8) modern versions, the new reference mecha-
       nism is generally easier to work with.  See below.

       Sometimes you don't want to pass the value of an array to a subroutine
       but rather the name of it, so that the subroutine can modify the global
       copy of it rather than working with a local copy.  In perl you can
       refer to all objects of a particular name by prefixing the name with a
       star: *foo.  This is often known as a "typeglob", because the star(1,4) on
       the front can be thought of as a wildcard match for all the funny pre-
       fix characters on variables and subroutines and such.

       When evaluated, the typeglob produces a scalar value that represents
       all the objects of that name, including any filehandle, format, or sub-
       routine.  When assigned to, it causes the name mentioned to refer to
       whatever "*" value was assigned to it.  Example:

           sub doubleary {
               local(*someary) = @_;
               foreach $elem (@someary) {
                   $elem *= 2;

       Scalars are already passed by reference, so you can modify scalar argu-
       ments without using this mechanism by referring explicitly to $_[0]
       etc.  You can modify all the elements of an array by passing all the
       elements as scalars, but you have to use the "*" mechanism (or the
       equivalent reference mechanism) to "push", "pop", or change the size of
       an array.  It will certainly be faster to pass the typeglob (or refer-

       Even if(3,n) you don't want to modify an array, this mechanism is useful for
       passing multiple arrays in(1,8) a single LIST, because normally the LIST
       mechanism will merge(1,8) all the array values so that you can't extract out
       the individual arrays.  For more on typeglobs, see "Typeglobs and File-
       handles" in(1,8) perldata.

       When to Still Use local()

       Despite the existence of "my", there are still three places where the
       "local" operator still shines.  In fact, in(1,8) these three places, you
       must use "local" instead of "my".

       1.  You need to give a global variable a temporary value, especially

           The global variables, like @ARGV or the punctuation variables, must
           be "local"ized with "local()".  This block reads in(1,8) /etc/motd, and
           splits it up into chunks separated by lines of equal signs, which
           are placed in(1,8) @Fields.

                   local @ARGV = ("/etc/motd");
                   local $/ = undef;
                   local $_ = <>;
                   @Fields = split(1,n) /^\s*=+\s*$/;

           It particular, it's important to "local"ize $_ in(1,8) any routine that
           assigns to it.  Look out for implicit assignments in(1,8) "while" condi-

       2.  You need to create a local file(1,n) or directory handle or a local

           A function that needs a filehandle of its own must use "local()" on
           a complete typeglob.   This can be used to create new symbol table

               sub ioqueue {
                   local  (*READER, *WRITER);    # not my!
                   pipe(2,8)    (READER,  WRITER)     or die "pipe: $!";
                   return (*READER, *WRITER);
               ($head, $tail) = ioqueue();

           See the Symbol module for a way to create anonymous symbol table

           Because assignment of a reference to a typeglob creates an alias,
           this can be used to create what is effectively a local function, or
           at least, a local alias.

                   local *grow = \&shrink; # only until this block exists
                   grow();                 # really calls shrink()
                   move(3x,7,3x curs_move)();                 # if(3,n) move(3x,7,3x curs_move)() grow()s, it shrink()s too
               grow();                     # get the real grow() again

           See "Function Templates" in(1,8) perlref for more about manipulating
           functions by name in(1,8) this way.

       3.  You want to temporarily change just one element of an array or

           You can "local"ize just one element of an aggregate.  Usually this
           is done on dynamics:

                   local $SIG{INT} = 'IGNORE';
                   funct();                            # uninterruptible
               # interruptibility automatically restored here

           But it also works on lexically declared aggregates.  Prior to
           5.005, this operation could on occasion misbehave.

       Pass by Reference

       If you want to pass more than one array or hash into a function--or
       return them from it--and have them maintain their integrity, then
       you're going to have to use an explicit pass-by-reference.  Before you
       do that, you need to understand references as detailed in(1,8) perlref.
       This section may not make much sense to you otherwise.

       Here are a few simple examples.  First, let's pass in(1,8) several arrays to
       a function and have it "pop" all of then, returning a new list of all
       their former last elements:

           @tailings = popmany ( \@a, \@b, \@c, \@d );

           sub popmany {
               my $aref;
               my @retlist = ();
               foreach $aref ( @_ ) {
                   push @retlist, pop @$aref;
               return @retlist;

       Here's how you might write(1,2) a function that returns a list of keys
       occurring in(1,8) all the hashes passed to it:

           @common = inter( \%foo, \%bar, \%joe );
           sub inter {
               my ($k, $href, %seen); # locals
               foreach $href (@_) {
                   while ( $k = each %$href ) {
               return grep { $seen{$_} == @_ } keys %seen;

       So far, we're using just the normal list return mechanism.  What hap-
       pens if(3,n) you want to pass or return a hash?  Well, if(3,n) you're using only
       one of them, or you don't mind them concatenating, then the normal
       calling convention is ok, although a little expensive.

       Where people get into trouble is here:

           (@a, @b) = func(@c, @d);
           (%a, %b) = func(%c, %d);

       That syntax simply won't work.  It sets just @a or %a and clears the @b
       or %b.  Plus the function didn't get passed into two separate arrays or
       hashes: it got one long list in(1,8) @_, as always.

       If you can arrange for everyone to deal with this through references,
       it's cleaner code, although not so nice(1,2) to look(1,8,3 Search::Dict) at.  Here's a function
       that takes two array references as arguments, returning the two array
       elements in(1,8) order of how many elements they have in(1,8) them:

           ($aref, $bref) = func(\@c, \@d);
           print "@$aref has more than @$bref\n";
           sub func {
               my ($cref, $dref) = @_;
               if(3,n) (@$cref > @$dref) {
                   return ($cref, $dref);
               } else {
                   return ($dref, $cref);

       It turns out that you can actually do this also:

           (*a, *b) = func(\@c, \@d);
           print "@a has more than @b\n";
           sub func {
               local (*c, *d) = @_;
               if(3,n) (@c > @d) {
                   return (\@c, \@d);
               } else {
                   return (\@d, \@c);

       Here we're using the typeglobs to do symbol table aliasing.  It's a tad
       subtle, though, and also won't work if(3,n) you're using "my" variables,
       because only globals (even in(1,8) disguise as "local"s) are in(1,8) the symbol

       If you're passing around filehandles, you could usually just use the
       bare typeglob, like *STDOUT, but typeglobs references work, too.  For

           sub splutter {
               my $fh = shift;
               print $fh "her um well a hmmm\n";

           $rec = get_rec(\*STDIN);
           sub get_rec {
               my $fh = shift;
               return scalar <$fh>;

       If you're planning on generating new filehandles, you could do this.
       Notice to pass back just the bare *FH, not its reference.

           sub openit {
               my $path = shift;
               local *FH;
               return open(2,3,n) (FH, $path) ? *FH : undef;


       Perl supports a very limited kind of compile-time argument checking
       using function prototyping.  If you declare

           sub mypush (\@@)

       then "mypush()" takes arguments exactly like "push()" does.  The func-
       tion declaration must be visible at compile time.  The prototype
       affects only interpretation of new-style calls to the function, where
       new-style is defined as not using the "&" character.  In other words,
       if(3,n) you call it like a built-in function, then it behaves like a built-
       in(1,8) function.  If you call it like an old-fashioned subroutine, then it
       behaves like an old-fashioned subroutine.  It naturally falls out from
       this rule that prototypes have no influence on subroutine references
       like "\&foo" or on indirect subroutine calls like "&{$subref}" or

       Method calls are not influenced by prototypes either, because the func-
       tion to be called is indeterminate at compile time(1,2,n), since the exact
       code called depends on inheritance.

       Because the intent of this feature is primarily to let you define sub-
       routines that work like built-in functions, here are prototypes for
       some other functions that parse almost exactly like the corresponding

           Declared as                 Called as

           sub mylink ($$)          mylink $old, $new
           sub myvec ($$$)          myvec $var, $offset, 1
           sub myindex ($$;$)       myindex &getstring, "substr"
           sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) - $off, $off
           sub myreverse (@)        myreverse $a, $b, $c
           sub myjoin ($@)          myjoin ":", $a, $b, $c
           sub mypop (\@)           mypop @array
           sub mysplice (\@$$@)     mysplice @array, @array, 0, @pushme
           sub mykeys (\%)          mykeys %{$hashref}
           sub myopen (*;$)         myopen HANDLE, $name
           sub mypipe (**)          mypipe READHANDLE, WRITEHANDLE
           sub mygrep (&@)          mygrep { /foo/ } $a, $b, $c
           sub myrand ($)           myrand 42
           sub mytime ()            mytime

       Any backslashed prototype character represents an actual argument that
       absolutely must start with that character.  The value passed as part of
       @_ will be a reference to the actual argument given in(1,8) the subroutine
       call, obtained by applying "\" to that argument.

       You can also backslash several argument types simultaneously by using
       the "\[]" notation:

           sub myref (\[$@%&*])

       will allow calling myref() as

           myref $var
           myref @array
           myref %hash
           myref &sub
           myref *glob(1,3,7,n)

       and the first argument of myref() will be a reference to a scalar, an
       array, a hash, a code, or a glob.

       Unbackslashed prototype characters have special meanings.  Any unback-
       slashed "@" or "%" eats all remaining arguments, and forces list con-
       text.  An argument represented by "$" forces scalar context.  An "&"
       requires an anonymous subroutine, which, if(3,n) passed as the first argu-
       ment, does not require the "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept(2,8) a bareword, constant, scalar
       expression, typeglob, or a reference to a typeglob in(1,8) that slot.  The
       value will be available to the subroutine either as a simple scalar, or
       (in(1,8) the latter two cases) as a reference to the typeglob.  If you wish
       to always convert such arguments to a typeglob reference, use Sym-
       bol::qualify_to_ref() as follows:

           use Symbol 'qualify_to_ref';

           sub foo (*) {
               my $fh = qualify_to_ref(shift, caller);

       A semicolon separates mandatory arguments from optional arguments.  It
       is redundant before "@" or "%", which gobble up everything else.

       Note how the last three examples in(1,8) the table above are treated spe-
       cially by the parser.  "mygrep()" is parsed as a true list operator,
       "myrand()" is parsed as a true unary operator with unary precedence the
       same as "rand(1,3)()", and "mytime()" is truly without arguments, just like
       "time(1,2,n)()".  That is, if(3,n) you say

           mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it would be
       parsed without a prototype.

       The interesting thing about "&" is that you can generate new syntax
       with it, provided it's in(1,8) the initial position:

           sub try (&@) {
               my($try,$catch) = @_;
               eval { &$try };
               if(3,n) ($@) {
                   local $_ = $@;
           sub catch (&) { $_[0] }

           try {
               die "phooey";
           } catch {
               /phooey/ and print "unphooey\n";

       That prints "unphooey".  (Yes, there are still unresolved issues having
       to do with visibility of @_.  I'm ignoring that question for the
       moment.  (But note that if(3,n) we make @_ lexically scoped, those anonymous
       subroutines can act like closures... (Gee, is this sounding a little
       Lispish?  (Never mind.))))

       And here's a reimplementation of the Perl "grep" operator:

           sub mygrep (&@) {
               my $code = shift;
               my @result;
               foreach $_ (@_) {
                   push(@result, $_) if(3,n) &$code;

       Some folks would prefer full alphanumeric prototypes.  Alphanumerics
       have been intentionally left out of prototypes for the express purpose
       of someday in(1,8) the future adding named(5,8), formal parameters.  The current
       mechanism's main goal is to let module writers provide better diagnos-
       tics for module users.  Larry feels the notation quite understandable
       to Perl programmers, and that it will not intrude greatly upon the meat
       of the module, nor make it harder to read.  The line noise is visually
       encapsulated into a small pill that's easy to swallow.

       If you try to use an alphanumeric sequence in(1,8) a prototype you will gen-
       erate an optional warning - "Illegal character in(1,8) prototype...".
       Unfortunately earlier versions of Perl allowed the prototype to be used
       as long as its prefix was a valid prototype.  The warning may be
       upgraded to a fatal error(8,n) in(1,8) a future version(1,3,5) of Perl once the majority
       of offending code is fixed.

       It's probably best to prototype new functions, not retrofit prototyping
       into older ones.  That's because you must be especially careful about
       silent impositions of differing list versus scalar contexts.  For exam-
       ple, if(3,n) you decide that a function should take just one parameter, like

           sub func ($) {
               my $n = shift;
               print "you gave me $n\n";

       and someone has been calling it with an array or expression returning a

           func( split(1,n) /:/ );

       Then you've just supplied an automatic "scalar" in(1,8) front of their argu-
       ment, which can be more than a bit surprising.  The old @foo which used
       to hold one thing doesn't get passed in.  Instead, "func()" now gets(3,n)
       passed in(1,8) a 1; that is, the number of elements in(1,8) @foo.  And the
       "split(1,n)" gets(3,n) called in(1,8) scalar context so it starts scribbling on your
       @_ parameter list.  Ouch!

       This is all very powerful, of course, and should be used only in(1,8) moder-
       ation to make the world a better place.

       Constant Functions

       Functions with a prototype of "()" are potential candidates for inlin-
       ing.  If the result after optimization and constant folding is either a
       constant or a lexically-scoped scalar which has no other references,
       then it will be used in(1,8) place of function calls made without "&".
       Calls made using "&" are never inlined.  (See for an easy
       way to declare most constants.)

       The following functions would all be inlined:

           sub pi ()           { 3.14159 }             # Not exact, but close.
           sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets(3,n),
                                                       # and it's inlined, too!
           sub ST_DEV ()       { 0 }
           sub ST_INO ()       { 1 }

           sub FLAG_FOO ()     { 1 << 8 }
           sub FLAG_BAR ()     { 1 << 9 }
           sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

           sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

           sub N () { int(OPT_BAZ) / 3 }

           sub FOO_SET () { 1 if(3,n) FLAG_MASK & FLAG_FOO }

       Be aware that these will not be inlined; as they contain inner scopes,
       the constant folding doesn't reduce them to a single constant:

           sub foo_set () { if(3,n) (FLAG_MASK & FLAG_FOO) { 1 } }

           sub baz_val () {
               if(3,n) (OPT_BAZ) {
                   return 23;
               else {
                   return 42;

       If you redefine a subroutine that was eligible for inlining, you'll get
       a mandatory warning.  (You can use this warning to tell whether or not
       a particular subroutine is considered constant.)  The warning is con-
       sidered severe enough not to be optional because previously compiled
       invocations of the function will still be using the old value of the
       function.  If you need to be able to redefine the subroutine, you need
       to ensure that it isn't inlined, either by dropping the "()" prototype
       (which changes calling semantics, so beware) or by thwarting the inlin-
       ing mechanism in(1,8) some other way, such as

           sub not_inlined () {
               23 if(3,n) $];

       Overriding Built-in Functions

       Many built-in functions may be overridden, though this should be tried
       only occasionally and for good reason.  Typically this might be done by
       a package attempting to emulate missing built-in functionality on a
       non-Unix system.

       Overriding may be done only by importing the name from a module at com-
       pile time--ordinary predeclaration isn't good enough.  However, the
       "use subs" pragma lets you, in(1,8) effect, predeclare subs via the import
       syntax, and these names may then override built-in ones:

           use subs 'chdir', 'chroot(1,2)', 'chmod(1,2)', 'chown(1,2)';
           chdir $somewhere;
           sub chdir { ... }

       To unambiguously refer to the built-in form, precede the built-in name
       with the special package qualifier "CORE::".  For example, saying
       "CORE::open()" always refers to the built-in "open(2,3,n)()", even if(3,n) the cur-
       rent package has imported some other subroutine called "&open(2,3,n)()" from
       elsewhere.  Even though it looks like a regular function call, it
       isn't: you can't take a reference to it, such as the incorrect
       "\&CORE::open" might appear to produce.

       Library modules should not in(1,8) general export built-in names like "open(2,3,n)"
       or "chdir" as part of their default @EXPORT list, because these may
       sneak into someone else's namespace and change the semantics unexpect-
       edly.  Instead, if(3,n) the module adds that name to @EXPORT_OK, then it's
       possible for a user to import the name explicitly, but not implicitly.
       That is, they could say

           use Module 'open(2,3,n)';

       and it would import the "open(2,3,n)" override.  But if(3,n) they said

           use Module;

       they would get the default imports without overrides.

       The foregoing mechanism for overriding built-in is restricted, quite
       deliberately, to the package that requests the import.  There is a sec-
       ond method that is sometimes applicable when you wish to override a
       built-in everywhere, without regard to namespace boundaries.  This is
       achieved by importing a sub into the special namespace
       "CORE::GLOBAL::".  Here is an example that quite brazenly replaces the
       "glob(1,3,7,n)" operator with something that understands regular expressions.

           package REGlob;
           require Exporter;
           @ISA = 'Exporter';
           @EXPORT_OK = 'glob(1,3,7,n)';

           sub import {
               my $pkg = shift;
               return unless @_;
               my $sym = shift;
               my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
               $pkg->export($where, $sym, @_);

           sub glob(1,3,7,n) {
               my $pat = shift;
               my @got;
               local *D;
               if(3,n) (opendir D, '.') {
                   @got = grep /$pat/, readdir(2,3) D;
                   closedir D;
               return @got;

       And here's how it could be (ab)used:

           #use REGlob 'GLOBAL_glob';      # override glob(1,3,7,n)() in(1,8) ALL namespaces
           package Foo;
           use REGlob 'glob(1,3,7,n)';              # override glob(1,3,7,n)() in(1,8) Foo:: only
           print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules

       The initial comment shows a contrived, even dangerous example.  By
       overriding "glob(1,3,7,n)" globally, you would be forcing the new (and subver-
       sive) behavior for the "glob(1,3,7,n)" operator for every namespace, without the
       complete cognizance or cooperation of the modules that own those names-
       paces.  Naturally, this should be done with extreme caution--if it must
       be done at all.

       The "REGlob" example above does not implement all the support needed to
       cleanly override perl's "glob(1,3,7,n)" operator.  The built-in "glob(1,3,7,n)" has dif-
       ferent behaviors depending on whether it appears in(1,8) a scalar or list
       context, but our "REGlob" doesn't.  Indeed, many perl built-in have
       such context sensitive behaviors, and these must be adequately sup-
       ported by a properly written override.  For a fully functional example
       of overriding "glob(1,3,7,n)", study the implementation of "File::DosGlob" in(1,8)
       the standard library.

       When you override a built-in, your replacement should be consistent (if(3,n)
       possible) with the built-in native syntax.  You can achieve this by
       using a suitable prototype.  To get the prototype of an overridable
       built-in, use the "prototype" function with an argument of
       "CORE::builtin_name" (see "prototype" in(1,8) perlfunc).

       Note however that some built-ins can't have their syntax expressed by a
       prototype (such as "system" or "chomp").  If you override them you
       won't be able to fully mimic their original syntax.

       The built-ins "do", "require" and "glob(1,3,7,n)" can also be overridden, but
       due to special magic(4,5), their original syntax is preserved, and you don't
       have to define a prototype for their replacements.  (You can't override
       the "do BLOCK" syntax, though).

       "require" has special additional dark magic: if(3,n) you invoke your
       "require" replacement as "require Foo::Bar", it will actually receive
       the argument "Foo/" in(1,8) @_.  See "require" in(1,8) perlfunc.

       And, as you'll have noticed from the previous example, if(3,n) you override
       "glob(1,3,7,n)", the "<*>" glob(1,3,7,n) operator is overridden as well.

       In a similar fashion, overriding the "readline" function also overrides
       the equivalent I/O operator "<FILEHANDLE>".

       Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.


       If you call a subroutine that is undefined, you would ordinarily get an
       immediate, fatal error(8,n) complaining that the subroutine doesn't exist.
       (Likewise for subroutines being used as methods, when the method
       doesn't exist in(1,8) any base class of the class's package.)  However, if(3,n)
       an "AUTOLOAD" subroutine is defined in(1,8) the package or packages used to
       locate the original subroutine, then that "AUTOLOAD" subroutine is
       called with the arguments that would have been passed to the original
       subroutine.  The fully qualified name of the original subroutine magi-
       cally appears in(1,8) the global $AUTOLOAD variable of the same package as
       the "AUTOLOAD" routine.  The name is not passed as an ordinary argument
       because, er, well, just because, that's why...

       Many "AUTOLOAD" routines load(7,n) in(1,8) a definition for the requested subrou-
       tine using eval(), then execute that subroutine using a special form of
       goto() that erases the stack frame of the "AUTOLOAD" routine without a
       trace.  (See the source to the standard module documented in(1,8)
       AutoLoader, for example.)  But an "AUTOLOAD" routine can also just emu-
       late the routine and never define it.   For example, let's pretend that
       a function that wasn't defined should just invoke "system" with those
       arguments.  All you'd do is:

           sub AUTOLOAD {
               my $program = $AUTOLOAD;
               $program =~ s/.*:://;
               system($program, @_);
           who('am', 'i');

       In fact, if(3,n) you predeclare functions you want to call that way, you
       don't even need parentheses:

           use subs qw(date who ls);
           who "am", "i";
           ls -l;

       A more complete example of this is the standard Shell module, which can
       treat undefined subroutine calls as calls to external programs.

       Mechanisms are available to help modules writers split(1,n) their modules
       into autoloadable files.  See the standard AutoLoader module described
       in(1,8) AutoLoader and in(1,8) AutoSplit, the standard SelfLoader modules in(1,8)
       SelfLoader, and the document on adding C functions to Perl code in(1,8) per-

       Subroutine Attributes

       A subroutine declaration or definition may have a list of attributes
       associated with it.  If such an attribute list is present, it is broken
       up at space or colon boundaries and treated as though a "use
       attributes" had been seen.  See attributes for details about what
       attributes are currently supported.  Unlike the limitation with the
       obsolescent "use attrs", the "sub : ATTRLIST" syntax works to associate
       the attributes with a pre-declaration, and not just with a subroutine

       The attributes must be valid as simple identifier names (without any
       punctuation other than the '_' character).  They may have a parameter
       list appended, which is only checked for whether its parentheses
       ('(',')') nest properly.

       Examples of valid syntax (even though the attributes are unknown):

           sub fnord (&\%) : switch(1,n)(10,foo(7,3))  :  expensive ;
           sub plugh () : Ugly('\(") :Bad ;
           sub xyzzy : _5x5 { ... }

       Examples of invalid syntax:

           sub fnord : switch(1,n)(10,foo() ; # ()-string not balanced
           sub snoid : Ugly('(') ;       # ()-string not balanced
           sub xyzzy : 5x5 ;             # "5x5" not a valid identifier
           sub plugh : Y2::north ;       # "Y2::north" not a simple identifier
           sub snurt : foo + bar ;       # "+" not a colon or space

       The attribute list is passed as a list of constant strings to the code
       which associates them with the subroutine.  In particular, the second
       example of valid syntax above currently looks like this in(1,8) terms of how
       it's parsed and invoked:

           use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

       For further details on attribute lists and their manipulation, see
       attributes and Attribute::Handlers.

       See "Function Templates" in(1,8) perlref for more about references and clo-
       sures.  See perlxs if(3,n) you'd like to learn about calling C subroutines
       from Perl.  See perlembed if(3,n) you'd like to learn about calling Perl
       subroutines from C.  See perlmod to learn about bundling up your func-
       tions in(1,8) separate files.  See perlmodlib to learn what library modules
       come standard on your system.  See perltoot to learn how to make object
       method calls.

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

References for this manual (incoming links)