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NAME    [Toc]    [Back]

       perlsub - Perl subroutines

SYNOPSIS    [Toc]    [Back]

       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

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

       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

       To import subroutines:

           use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

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

DESCRIPTION    [Toc]    [Back]

       Like many languages, Perl provides for user-defined subroutines.
  These may be located anywhere in the main program,
 loaded in 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 functions 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 these call and return lists will collapse,
       losing their identities--but you may always use pass-byreference
 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 without an explicit return
       statement is called a subroutine, but there's really no
       difference from Perl's perspective.)
       Any arguments passed in show up in the array @_.  Therefore,
 if you called a function with two arguments, those
       would be stored in $_[0] and $_[1].  The array @_ is a
       local array, but its elements are aliases for the actual
       scalar parameters.  In particular, if an element $_[0] is
       updated, the corresponding argument is updated (or an
       error occurs if 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 created only when
       (and if) 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
       any arguments.

       The return value of a subroutine is the value of the last
       expression evaluated by that sub, or the empty list in the
       case of an empty sub.  More explicitly, a "return" statement
 may be used to exit the subroutine, optionally specifying
 the returned value, which will be evaluated in 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 list
       context, the undefined value in scalar context, or nothing
       in 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 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 of
       functions in a separate package (and probably a separate
       file), see "Packages" in perlmod.


           sub max {
               my $max = shift(@_);
               foreach $foo (@_) {
                   $max = $foo if $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 ($lookahead =~ /^[ ]/) {
                       $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

           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 inplace
 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 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 the "upcase_in()" function were
       written to return a copy of its parameters instead of
       changing them in 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 @_.
       This is one area where Perl's simple argument-passing
       style shines.  The "upcase()" function would work perfectly
 well without changing the "upcase()" definition
       even if we fed it things like this:

           @newlist   = upcase(@list1, @list2);
           @newlist   = upcase( split /:/, $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 @a and made @b empty.
       See "Pass by Reference" for alternatives.

       A subroutine may be called using an explicit "&" prefix.
       The "&" is optional in modern Perl, as are parentheses if
       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 subroutine name or reference using the "&$subref()"
 or "&{$subref}()" constructs, 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 omitted, no @_ array is set up for the subroutine:
       the @_ array at the time of the call is visible to subroutine
 instead.  This is an efficiency mechanism that new
       users 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
       you know what you're doing.  See Prototypes below.

       Subroutines whose names are in all upper case are reserved
       to the Perl core, as are modules whose names are in all
       lower case.  A subroutine in all capitals is a looselyheld
 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 perltie and PerlIO::via.

       The "BEGIN", "CHECK", "INIT" and "END" subroutines are not
       so much subroutines as named special code blocks, of which
       you can have more than one in a package, and which you can
       not call explicitely.  See "BEGIN, CHECK, INIT and END" in

       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
           my @oof = @bar;     # declare @oof lexical,  and  init
           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
 confined to the enclosing block, conditional
       ("if/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 parentheses.  All
       listed elements must be legal lvalues.  Only alphanumeric
       identifiers may be lexically scoped--magical built-ins
       like $/ must currently be "local"ized with "local"

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

       This doesn't mean that a "my" variable declared in a statically
 enclosing lexical scope would be invisible.  Only
       dynamic scopes are cut off.   For example, the "bumpx()"
       function below has access to the lexical $x variable
       because both the "my" and the "sub" occurred at the same
       scope, presumably file scope.
           my $x = 10;
           sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the
       scope it is being evaluated in, 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 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 $n0;
        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 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 variable:

           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
       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 the conditional

           if ((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 perlsyn for information on the scope of variables
 in statements with modifiers.

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

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

       the scope of $i extends to the end of the loop, but not
       beyond it, rendering the value of $i inaccessible within

       Some users 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 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 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, 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 perlref.  Actual initialization is delayed
       until run time, though, so it gets executed at the appropriate
 time, such as each time through a loop, for example.

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

           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 $::x0;

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of a
       file to hide any such identifiers from the world outside
       that file.  This is similar in spirit to C's static variables
 when they are used at the file level.  To do this
       with a subroutine requires the use of a closure (an anonymous
 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 any package's symbol
       table.  Remember that it's not REALLY called
       $some_pack::secret_version or anything; it's just
       $secret_version, unqualified and unqualifiable.

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

       Persistent Private Variables    [Toc]    [Back]

       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,
       but with implicit garbage collection.

       Unlike local variables in C or C++, Perl's lexical variables
 don't necessarily get recycled just because their
       scope has exited.  If something 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 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
       an extra block, and put the static variable outside the
       function but in the block.

               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

       If this function is being sourced in from a separate file
       via "require" or "use", then this is probably just fine.
       If it's all in 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 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 perlmod about the special
 triggered code blocks, "BEGIN", "CHECK", "INIT" and

       If declared at the outermost scope (the file scope), then
       lexicals work somewhat like C's file statics.  They are
       available to all functions in that same file declared
       below them, but are inaccessible from outside that file.
       This strategy is sometimes used in modules to create private
 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 subroutines.


           # localization of values

           local  $foo;                  #  make $foo dynamically
           local (@wid, %get);         # make list  of  variables
           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
                                       # 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  =  andal;    # 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 sub-
       routine 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 declarations.

       Some types of lvalues can be localized as well : hash and
       array elements 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 parentheses.  This operator
 works by saving the current values of those variables
       in 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 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 executed
       each time 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 won't go away.  That means that
       all side-effects related to this magic still work with the
       localized value.

       This feature allows code like this to work :

           # Read the whole contents of FILE in $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 Modification of a read-only
       value attempted, because the $1 variable is magical and
       read-only :

           local $1 = 2;

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

           sub f { local $_ = "foo"; print }
           for ($1) {
               # now $_ is aliased to $1, thus is magic 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 a
       future release of Perl; in the meantime, avoid code that
       relies on any particular behaviour of localising tied
       arrays or hashes (localising individual elements is still
       okay).  See "Localising Tied Arrays and Hashes Is Broken"
       in perl58delta for more details.

       Localization of globs

       The construct

           local *name;

       creates a whole new symbol table entry for the glob "name"
       in the current package.  That means that all variables in
       its glob slot ($name, @name, %name, &name, and the "name"
       filehandle) are dynamically reset.

       This implies, among other things, that any magic 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 separator.

       Notably, if you want to work with a brand new value of the
       default scalar $_, and avoid the potential problem listed
       above about $_ previously carrying a magic 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 element).  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 in the "local()", or the
       array element whose index was named in the "local()".  If
       that element was deleted while the "local()" was in 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 the skipped elements
       with "undef".  For instance, if 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 . . .0;
                    last unless $e > 3;
                if (@ary) {
                    $hash{'only a'} = 'test';
                    delete $hash{'a'};
           print join(' ', map  {  "$_  $hash{$_}"  }  sort  keys
           print "The array has ",scalar(@ary)," elements: ",
                 join(',  ',  map  {  defined $_ ? $_ : 'undef' }

       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 future.

       Lvalue subroutines    [Toc]    [Back]

       WARNING: Lvalue subroutines are still experimental and the
       implementation may change in 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 the subroutine
 call is replaced by a scalar. For example, consider:

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

       Both subroutines here are called in 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 a list context.

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

           You can't use the return keyword, you must pass out
           the value before falling out of subroutine scope. (see
           comment in 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 way out.

           They violate encapsulation.  A normal mutator can
           check the supplied argument before setting the
           attribute it is protecting, an lvalue subroutine never
           gets 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 this section was originally
 the only way to simulate pass-by-reference in older
       versions of Perl.  While it still works fine in modern
       versions, the new reference mechanism 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 on the front can be thought of as a wildcard
       match for all the funny prefix 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 subroutine.  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 arguments 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 reference).

       Even if you don't want to modify an array, this mechanism
       is useful for passing multiple arrays in a single LIST,
       because normally the LIST mechanism will merge all the
       array values so that you can't extract out the individual
       arrays.  For more on typeglobs, see "Typeglobs and Filehandles"
 in 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 these three places, you must use "local" instead of

       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 /etc/motd, and splits it up into chunks
           separated by lines of equal signs, which are placed in

                   local @ARGV = ("/etc/motd");
                   local $/ = undef;
                   local $_ = <>;
                   @Fields = split /^=+$/;

           It particular, it's important to "local"ize $_ in any
           routine that assigns to it.  Look out for implicit
           assignments in "while" conditionals.

       2.  You need to create a local file or directory handle or
           a local function.

           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 entries:

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

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

           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

                   local  *grow = shrink; # only until this block
                   grow();                   #    really    calls
                   move();                  #  if move() grow()s,
it shrink()s too
               grow();                     # get the real  grow()

           See "Function Templates" in perlref for more about
           manipulating functions by name in this way.
       3.  You want to temporarily change just one element of an
           array or hash.

           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    [Toc]    [Back]

       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 perlref.  This
       section may not make much sense to you otherwise.

       Here are a few simple examples.  First, let's pass in 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 a function that returns a list
       of keys occurring in 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 happens if you want to pass or return a hash?  Well,
       if 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 @_, as always.

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

           ($aref, $bref) = func(@c, @d);
           print "@$aref has more than @$bref0;
           sub func {
               my ($cref, $dref) = @_;
               if (@$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 @b0;
           sub func {
               local (*c, *d) = @_;
               if (@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
       you're using "my" variables, because only globals (even in
       disguise as "local"s) are in the symbol table.

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

           sub splutter {
               my $fh = shift;
               print $fh "her um well a hmmm0;

           $rec = get_rec(TDIN);
           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 (FH, $path) ? *FH : undef;

       Prototypes    [Toc]    [Back]

       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 function declaration must be visible at compile
       time.  The prototype affects only interpretation of newstyle
 calls to the function, where new-style is defined as
       not using the "&" character.  In other words, if you call
       it like a built-in function, then it behaves like a builtin
 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 "&{$subref}" or "$subref->()".

       Method calls are not influenced by prototypes either,
       because the function to be called is indeterminate at compile
 time, since the exact code called depends on inheritance.

       Because the intent of this feature is primarily to let you
       define subroutines that work like built-in functions, here
       are prototypes for some other functions that parse almost
       exactly like the corresponding built-in.
           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,
           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 the subroutine call, obtained by
       applying "

       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

       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 unbackslashed "@" or "%" eats all remaining arguments,
       and forces list context.  An argument represented by "$"
       forces scalar context.  An "&" requires an anonymous subroutine,
 which, if passed as the first argument, does not
       require the "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept a bareword, constant,
 scalar expression, typeglob, or a reference to a
       typeglob in that slot.  The value will be available to the
       subroutine either as a simple scalar, or (in the latter
       two cases) as a reference to the typeglob.  If you wish to
       always convert such arguments to a typeglob reference, use
       Symbol::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 the table above are
       treated specially 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()", and
       "mytime()" is truly without arguments, just like "time()".
       That is, if 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 the initial position:

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

           try {
               die "phooey";
           } catch {
               /phooey/ and print "unphooey0;

       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 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, $_) if &$code;

       Some folks would prefer full alphanumeric prototypes.
       Alphanumerics have been intentionally left out of prototypes
 for the express purpose of someday in the future
       adding named, formal parameters.  The current mechanism's
       main goal is to let module writers provide better diagnostics
 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 a prototype
       you will generate an optional warning - "Illegal character
       in 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 in a future version 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 example, if
       you decide that a function should take just one parameter,
       like this:

           sub func ($) {
               my $n = shift;
               print "you gave me $n0;

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

           func( split /:/ );

       Then you've just supplied an automatic "scalar" in front
       of their argument, 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 passed in a 1;
       that is, the number of elements in @foo.  And the "split"
       gets called in scalar context so it starts scribbling on
       your @_ parameter list.  Ouch!
       This is all very powerful, of course, and should be used
       only in moderation to make the world a better place.

       Constant Functions    [Toc]    [Back]

       Functions with a prototype of "()" are potential candidates
 for inlining.  If the result after optimization and
       constant folding is either a constant or a lexicallyscoped
 scalar which has no other references, then it will
       be used in place of function calls made without "&".
       Calls made using "&" are never inlined.  (See constant.pm
       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,
                                                       # 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 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 (FLAG_MASK & FLAG_FOO) { 1 } }

           sub baz_val () {
               if (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 considered 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 inlining
       mechanism in some other way, such as

           sub not_inlined () {
               23 if $];

       Overriding Built-in Functions    [Toc]    [Back]

       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 compile time--ordinary predeclaration isn't good
       enough.  However, the "use subs" pragma lets you, in
       effect, predeclare subs via the import syntax, and these
       names may then override built-in ones:

           use subs 'chdir', 'chroot', 'chmod', 'chown';
           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()", even if the current package has
       imported some other subroutine called "&open()" 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 general export built-in
       names like "open" or "chdir" as part of their default
       @EXPORT list, because these may sneak into someone else's
       namespace and change the semantics unexpectedly.  Instead,
       if 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';

       and it would import the "open" override.  But if 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 second 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" operator with something that understands
 regular expressions.

           package REGlob;
           require Exporter;
           @ISA = 'Exporter';
           @EXPORT_OK = 'glob';

           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 {
               my $pat = shift;
               my @got;
               local *D;
               if (opendir D, '.') {
                   @got = grep /$pat/, readdir D;
                   closedir D;
               return @got;

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

           #use  REGlob  'GLOBAL_glob';      # override glob() in
ALL namespaces
           package Foo;
           use REGlob 'glob';              # override  glob()  in
Foo:: only
           print  for <^[a-z_]+.pm;     # show all pragmatic modules

       The initial comment shows a contrived, even dangerous
       example.  By overriding "glob" globally, you would be
       forcing the new (and subversive) behavior for the "glob"
       operator for every namespace, without the complete cognizance
 or cooperation of the modules that own those
       namespaces.  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" operator.
       The built-in "glob" has different behaviors depending on
       whether it appears in 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
       supported by a properly written override.  For a fully
       functional example of overriding "glob", study the implementation
 of "File::DosGlob" in the standard library.

       When you override a built-in, your replacement should be
       consistent  (if 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 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" can also be overridden,
 but due to special magic, 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 you invoke
       your "require" replacement as "require Foo::Bar", it will
       actually receive the argument "Foo/Bar.pm" in @_.  See
       "require" in perlfunc.

       And, as you'll have noticed from the previous example, if
       you override "glob", the "<*>" glob 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

       Autoloading    [Toc]    [Back]

       If you call a subroutine that is undefined, you would
       ordinarily get an immediate, fatal error complaining that
       the subroutine doesn't exist.  (Likewise for subroutines
       being used as methods, when the method doesn't exist in
       any base class of the class's package.)  However, if an
       "AUTOLOAD" subroutine is defined in 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 magically
       appears in 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 in a definition for the
       requested subroutine 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
       AutoLoader, for example.)  But an "AUTOLOAD" routine can
       also just emulate 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 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
       their modules into autoloadable files.  See the standard
       AutoLoader module described in AutoLoader and in
       AutoSplit, the standard SelfLoader modules in SelfLoader,
       and the document on adding C functions to Perl code in

       Subroutine Attributes    [Toc]    [Back]

       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 definition.

       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

       Examples of valid syntax (even though the attributes are

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

       Examples of invalid syntax:

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

       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 terms of how it's parsed and

           use attributes __PACKAGE__, plugh, q[Ugly('], 'Bad';

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

SEE ALSO    [Toc]    [Back]

       See "Function

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