 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 predeclared/imported.
  &NAME(LIST);   # Circumvent prototypes.
  &NAME;   # Makes current @_ visible to called subroutine.

DESCRIPTION

Like many languages, Perl provides for user-defined subroutines.These may be located anywhere in the main program, loaded in fromother files via the do, require, or use keywords, orgenerated on the fly using eval or anonymous subroutines.You can even call a function indirectly using a variable containingits name or a CODE reference.

The Perl model for function call and return values is simple: allfunctions are passed as parameters one single flat list of scalars, andall functions likewise return to their caller one single flat list ofscalars. Any arrays or hashes in these call and return lists willcollapse, losing their identities--but you may always usepass-by-reference instead to avoid this. Both call and return lists maycontain as many or as few scalar elements as you'd like. (Often afunction without an explicit return statement is called a subroutine, butthere's really no difference from Perl's perspective.)

Any arguments passed in show up in the array @_. Therefore, ifyou called a function with two arguments, those would be stored in$_[0] and $_[1]. The array @_ is a local array, but itselements are aliases for the actual scalar parameters. In particular,if an element $_[0] is updated, the corresponding argument isupdated (or an error occurs if it is not updatable). If an argumentis an array or hash element which did not exist when the functionwas called, that element is created only when (and if) it is modifiedor a reference to it is taken. (Some earlier versions of Perlcreated the element whether or not the element was assigned to.)Assigning to the whole array @_ removes that aliasing, and doesnot update any arguments.

A return statement may be used to exit a subroutine, optionallyspecifying the returned value, which will be evaluated in theappropriate context (list, scalar, or void) depending on the context ofthe subroutine call. If you specify no return value, the subroutinereturns an empty list in list context, the undefined value in scalarcontext, or nothing in void context. If you return one or moreaggregates (arrays and hashes), these will be flattened together intoone large indistinguishable list.

If no return is found and if the last statement is an expression, itsvalue is returned. If the last statement is a loop control structurelike a foreach or a while, the returned value is unspecified. Theempty sub returns the empty list.

Perl does not have named formal parameters. In practice all youdo is assign to a my() list of these. Variables that aren'tdeclared to be private are global variables. For gory detailson creating private variables, see Private Variables via my()and Temporary Values via local(). To create protectedenvironments for a set of functions in a separate package (andprobably a separate file), see Packages in perlmod.

Example:

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

Example:

 # get a line, combining continuation lines
  #  that start with whitespace
 
  sub get_line {
 $thisline = $lookahead;  # global variables!
 LINE: while (defined($lookahead = <STDIN>)) {
  if ($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 effectof turning call-by-reference into call-by-value. Otherwise afunction is free to do in-place modifications of @_ and changeits 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 anargument were actually literal and you tried to change it, you'd take a(presumably fatal) exception. For example, this won't work:

 upcase_in("frederick");

It would be much safer if the upcase_in() functionwere written to return a copy of its parameters insteadof 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 waspassed real scalars or arrays. Perl sees all arguments as one big,long, flat parameter list in @_. This is one area wherePerl'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 alsoflattened on return. So all you have managed to do here is storedeverything 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 thesubroutine has been predeclared. The & is not optionalwhen just naming the subroutine, such as when it's used asan argument to defined() or undef(). Nor is it optional when youwant to do an indirect subroutine call with a subroutine name orreference 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 calledusing the & form, the argument list is optional, and if omitted,no @_ array is set up for the subroutine: the @_ array at thetime of the call is visible to subroutine instead. This is anefficiency 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 alsodisables any prototype checking on arguments you do provide. Thisis partly for historical reasons, and partly for having a convenient wayto cheat if you know what you're doing. See Prototypes below.

Since Perl 5.16.0, the __SUB__ token is available under use feature'current_sub' and use 5.16.0. It will evaluate to a reference to thecurrently-running sub, which allows for recursive calls without knowingyour subroutine's name.

 use 5.16.0;
  my $factorial = sub {
   my ($x) = @_;
   return 1 if $x == 1;
   return($x * __SUB__->( $x - 1 ) );
  };

Subroutines whose names are in all upper case are reserved to the Perlcore, as are modules whose names are in all lower case. A subroutine inall capitals is a loosely-held convention meaning it will be calledindirectly 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, UNITCHECK, CHECK, INIT and END subroutinesare not so much subroutines as named special code blocks, of which youcan have more than one in a package, and which you can not callexplicitly. See BEGIN, UNITCHECK, CHECK, INIT and END in perlmod

Private Variables via my()

Synopsis:

 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 stillevolving. The current semantics and interface are subject to change.See attributes and Attribute::Handlers.

The my operator declares the listed variables to be lexicallyconfined 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, thelist must be placed in parentheses. All listed elements must belegal lvalues. Only alphanumeric identifiers may be lexicallyscoped--magical built-ins like $/ must currently be localizedwith local instead.

Unlike dynamic variables created by the local operator, lexicalvariables declared with my are totally hidden from the outsideworld, including any called subroutines. This is true if it's thesame subroutine called from itself or elsewhere--every call getsits own copy.

This doesn't mean that a my variable declared in a staticallyenclosing lexical scope would be invisible. Only dynamic scopesare cut off. For example, the bumpx() function below has accessto the lexical $x variable because both the my and the suboccurred at the same scope, presumably file scope.

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

An eval(), however, can see lexical variables of the scope it isbeing evaluated in, so long as the names aren't hidden by declarations withinthe eval() itself. See perlref.

The parameter list to my() may be assigned to if desired, which allows youto initialize your variables. (If no initializer is given for aparticular variable, it is created with the undefined value.) Commonlythis 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 whenyou do assign to variables in its argument list, my doesn'tchange 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 afterthe current statement. Thus,

 my $x = $x;

can be used to initialize a new $x with the value of the old $x, andthe 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 thebraces that delimit their controlled blocks; control expressions arepart 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 ofthe loop construct (including the continue clause), but not beyondit. Similarly, in the conditional

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

the scope of $answer extends from its declaration through the restof that conditional, including any elsif and else clauses, but not beyond it. See Simple Statements in perlsyn for informationon the scope of variables in statements with modifiers.

The foreach loop defaults to scoping its index variable dynamicallyin the manner of local. However, if the index variable isprefixed with the keyword my, or if there is already a lexicalby that name in scope, then a new lexical is created instead. Thusin the loop

 for my $i (1, 2, 3) {
  some_function();
  }

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

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 enclosingblock must either refer to a lexical variable, be predeclared viaour or use vars, or else must be fully qualified with the package name.A compilation error results otherwise. An inner block may countermandthis with no strict 'vars'.

A my has both a compile-time and a run-time effect. At compiletime, the compiler takes notice of it. The principal usefulnessof this is to quiet use strict 'vars', but it is also essentialfor generation of closures as detailed in perlref. Actualinitialization is delayed until run time, though, so it gets executedat the appropriate time, such as each time through a loop, forexample.

Variables declared with my are not part of any package and are thereforenever fully qualified with the package name. In particular, you're notallowed to try to make a package variable (or other global) lexical:

 my $pack::var;# ERROR!  Illegal syntax

In fact, a dynamic variable (also known as package or global variables)are still accessible using the fully qualified :: notation even while alexical 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 fileto hide any such identifiers from the world outside that file. Thisis similar in spirit to C's static variables when they are used atthe file level. To do this with a subroutine requires the use ofa closure (an anonymous function that accesses enclosing lexicals).If you want to create a private subroutine that cannot be calledfrom outside that block, it can declare a lexical variable containingan anonymous sub reference:

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

As long as the reference is never returned by any function within themodule, no outside module can see the subroutine, because its name is not inany 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 methodshave to be in the symbol table of some package to be found. SeeFunction Templates in perlref for something of a work-around tothis.

Persistent Private Variables

There are two ways to build persistent private variables in Perl 5.10.First, you can simply use the state feature. Or, you can use closures,if you want to stay compatible with releases older than 5.10.

Persistent variables via state()

Beginning with Perl 5.9.4, you can declare variables with the statekeyword in place of my. For that to work, though, you must haveenabled that feature beforehand, either by using the feature pragma, orby using -E on one-liners (see feature). Beginning with Perl 5.16,the CORE::state form does not require thefeature pragma.

For example, the following code maintains a private counter, incrementedeach time the gimme_another() function is called:

 use feature 'state';
  sub gimme_another { state $x; return ++$x }

Also, since $x is lexical, it can't be reached or modified by any Perlcode outside.

When combined with variable declaration, simple scalar assignment to statevariables (as in state $x = 42) is executed only the first time. When suchstatements are evaluated subsequent times, the assignment is ignored. Thebehavior of this sort of assignment to non-scalar variables is undefined.

Persistent variables with closures

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

Unlike local variables in C or C++, Perl's lexical variables don'tnecessarily get recycled just because their scope has exited.If something more permanent is still aware of the lexical, it willstick around. So long as something else references a lexical, thatlexical won't be freed--which is as it should be. You wouldn't wantmemory being free until you were done using it, or kept around once youwere done. Automatic garbage collection takes care of this for you.

This means that you can pass back or save away references to lexicalvariables, 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 amechanism for giving a function private variables with both lexicalscoping and a static lifetime. If you do want to create something likeC'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 gimme_another

If this function is being sourced in from a separate filevia require or use, then this is probably just fine. If it'sall in the main program, you'll need to arrange for the myto be executed early, either by putting the whole block aboveyour main program, or more likely, placing merely a BEGINcode block around it to make sure it gets executed before your programstarts to run:

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

See BEGIN, UNITCHECK, CHECK, INIT and END in perlmod about thespecial triggered code blocks, BEGIN, UNITCHECK, CHECK,INIT and END.

If declared at the outermost scope (the file scope), then lexicalswork somewhat like C's file statics. They are available to allfunctions in that same file declared below them, but are inaccessiblefrom outside that file. This strategy is sometimes used in modulesto create private variables that the whole module can see.

Temporary Values via local()

WARNING: In general, you should be using my instead of local, becauseit's faster and safer. Exceptions to this include the global punctuationvariables, global filehandles and formats, and direct manipulation of thePerl symbol table itself. local is mostly used when the current valueof a variable must be visible to called subroutines.

Synopsis:

 # 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
  delete local $hash{key}; # delete this entry for the current block
  local ($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 theenclosing block, eval, or do FILE--and to any subroutinecalled from within that block. A local just gives temporaryvalues to global (meaning package) variables. It does not createa local variable. This is known as dynamic scoping. Lexical scopingis done with my, which works more like C's auto declarations.

Some types of lvalues can be localized as well: hash and array elementsand slices, conditionals (provided that their result is alwayslocalizable), and symbolic references. As for simple variables, thiscreates new, dynamically scoped values.

If more than one variable or expression is given to local, they must beplaced in parentheses. This operator worksby saving the current values of those variables in its argument list on ahidden stack and restoring them upon exiting the block, subroutine, oreval. This means that called subroutines can also reference the localvariable, but not the global one. The argument list may be assigned to ifdesired, which allows you to initialize your local variables. (If noinitializer is given for a particular variable, it is created with anundefined value.)

Because local is a run-time operator, it gets executed each timethrough a loop. Consequently, it's more efficient to localize yourvariables outside the loop.

Grammatical note on local()

A local is simply a modifier on an lvalue expression. When you assign toa localized variable, the local doesn't change whether its list is viewedas 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 relatedto 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 ; forexample, the following statement dies, as of perl 5.9.0, with an errorModification of a read-only value attempted, because the $1 variable ismagical and read-only :

 local $1 = 2;

One exception is the default scalar variable: starting with perl 5.14local($_) will always strip all magic from $_, to make it possibleto safely reuse $_ in a subroutine.

WARNING: Localization of tied arrays and hashes does not currentlywork as described.This will be fixed in a future release of Perl; in the meantime, avoidcode that relies on any particular behaviour of localising tied arraysor hashes (localising individual elements is still okay).See Localising Tied Arrays and Hashes Is Broken in perl58delta for moredetails.

Localization of globs

The construct

 local *name;

creates a whole new symbol table entry for the glob name in thecurrent 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 bythose variables is locally lost. In other words, saying local */will not have any effect on the internal value of the input recordseparator.

Localization of elements of composite types

It's also worth taking a moment to explain what happens when youlocalize a member of a composite type (i.e. an array or hash element).In this case, the element is localized by name. This means thatwhen the scope of the local() ends, the saved value will berestored to the hash element whose key was named in the local(), orthe array element whose index was named in the local(). If thatelement was deleted while the local() was in effect (e.g. by adelete() from a hash or a shift() of an array), it will springback into existence, possibly extending an array and filling in theskipped 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 . . .\n";
  last unless $e > 3;
  }
  if (@ary) {
  $hash{'only a'} = 'test';
  delete $hash{'a'};
  }
  }
  print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
  print "The array has ",scalar(@ary)," elements: ",
   join(', ', 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 compositetypes is subject to change in future.

Localized deletion of elements of composite types

You can use the delete local $array[$idx] and delete local $hash{key}constructs to delete a composite type entry for the current block and restoreit when it ends. They return the array/hash value before the localization,which means that they are respectively equivalent to

 do {
  my $val = $array[$idx];
  local  $array[$idx];
  delete $array[$idx];
  $val
  }

and

 do {
  my $val = $hash{key};
  local  $hash{key};
  delete $hash{key};
  $val
  }

except that for those the local is scoped to the do block. Slices arealso accepted.

 my %hash = (
  a => [ 7, 8, 9 ],
  b => 1,
  )
 
  {
  my $a = delete local $hash{a};
  # $a is [ 7, 8, 9 ]
  # %hash is (b => 1)
 
  {
   my @nums = delete local @$a[0, 2]
   # @nums is (7, 9)
   # $a is [ undef, 8 ]
 
   $a[0] = 999; # will be erased when the scope ends
  }
  # $a is back to [ 7, 8, 9 ]
 
  }
  # %hash is back to its original state

Lvalue subroutines

WARNING: Lvalue subroutines are still experimental and theimplementation 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 {
 $val;  # or:  return $val;
  }
  sub nomod {
 $val;
  }
 
  canmod() = 5;   # assigns to $val
  nomod()  = 5;   # ERROR

The scalar/list context for the subroutine and for the right-handside of assignment is determined as if the subroutine call is replacedby 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 is at least one reason to becircumspect.

They violate encapsulation. A normal mutator can check the suppliedargument before setting the attribute it is protecting, an lvaluesubroutine 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
 $some_array_ref;
  }
 
  # set_arr_lv cannot stop this !
  set_arr_lv() = { a => 1 };

Passing Symbol Table Entries (typeglobs)

WARNING: The mechanism described in this section was originallythe only way to simulate pass-by-reference in older versions ofPerl. While it still works fine in modern versions, the new referencemechanism is generally easier to work with. See below.

Sometimes you don't want to pass the value of an array to a subroutinebut rather the name of it, so that the subroutine can modify the globalcopy of it rather than working with a local copy. In perl you canrefer to all objects of a particular name by prefixing the namewith a star: *foo. This is often known as a "typeglob", because thestar on the front can be thought of as a wildcard match for all thefunny prefix characters on variables and subroutines and such.

When evaluated, the typeglob produces a scalar value that representsall the objects of that name, including any filehandle, format, orsubroutine. When assigned to, it causes the name mentioned to refer towhatever * value was assigned to it. Example:

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

Scalars are already passed by reference, so you can modifyscalar arguments without using this mechanism by referring explicitlyto $_[0] etc. You can modify all the elements of an array by passingall the elements as scalars, but you have to use the * mechanism (orthe equivalent reference mechanism) to push, pop, or change the size ofan 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 forpassing multiple arrays in a single LIST, because normally the LISTmechanism will merge all the array values so that you can't extract outthe individual arrays. For more on typeglobs, seeTypeglobs and Filehandles in perldata.

When to Still Use local()

Despite the existence of my, there are still three places where thelocal operator still shines. In fact, in these three places, youmust 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 localized with local(). This block reads in /etc/motd, and splitsit up into chunks separated by lines of equal signs, which are placedin @Fields.

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

It particular, it's important to localize $_ in any routine that assignsto 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 uselocal() on a complete typeglob. This can be used to create new symboltable 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 tableentries.

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

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

See Function Templates in perlref for more about manipulatingfunctions by name in this way.

3.

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

You can localize just one element of an aggregate. Usually thisis 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--orreturn them from it--and have them maintain their integrity, thenyou're going to have to use an explicit pass-by-reference. Before youdo 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 arraysto a function and have it pop all of then, returning a new listof 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 alist 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 ) {
 $seen{$k}++;
  }
 }
 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 themconcatenating, then the normal calling convention is ok, althougha little expensive.

Where people get into trouble is here:

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

That syntax simply won't work. It sets just @a or %a andclears the @b or %b. Plus the function didn't get passedinto 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'scleaner code, although not so nice to look at. Here's a function thattakes two array references as arguments, returning the two array elementsin order of how many elements they have in them:

 ($aref, $bref) = func(\@c, \@d);
  print "@$aref has more than @$bref\n";
  sub func {
 my ($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 @b\n";
  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'sa tad subtle, though, and also won't work if you're using myvariables, because only globals (even in disguise as locals)are in the symbol table.

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

 splutter(\*STDOUT);
  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 (FH, $path) ? *FH : undef;
  }

Prototypes

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

 sub mypush (+@)

then mypush() takes arguments exactly like push() does. Thefunction declaration must be visible at compile time. The prototypeaffects only interpretation of new-style calls to the function,where new-style is defined as not using the & character. Inother words, if you call it like a built-in function, then it behaveslike a built-in function. If you call it like an old-fashionedsubroutine, then it behaves like an old-fashioned subroutine. Itnaturally falls out from this rule that prototypes have no influenceon subroutine references like \&foo or on indirect subroutinecalls like &{$subref} or $subref->().

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

Because the intent of this feature is primarily to let you definesubroutines that work like built-in functions, here are prototypesfor some other functions that parse almost exactly like thecorresponding built-in.

 Declared asCalled 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, 0, 2, @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 argumentthat must start with that character (optionally preceded by my,our or local), with the exception of $, which willaccept any scalar lvalue expression, such as $foo = 7 ormy_function()->[0]. The value passed as part of @_ will be areference to the actual argument given in the subroutine call,obtained by applying \ to that argument.

You can use the \[] backslash group notation to specify more than oneallowed argument type. For example:

 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 toa scalar, an array, a hash, a code, or a glob.

Unbackslashed prototype characters have special meanings. Anyunbackslashed @ or % eats all remaining arguments, and forceslist context. An argument represented by $ forces scalar context. An& requires an anonymous subroutine, which, if passed as the firstargument, 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 beavailable to the subroutine either as a simple scalar, or (in the lattertwo cases) as a reference to the typeglob. If you wish to always convertsuch arguments to a typeglob reference, use Symbol::qualify_to_ref() asfollows:

 use Symbol 'qualify_to_ref';
 
  sub foo (*) {
 my $fh = qualify_to_ref(shift, caller);
 ...
  }

The + prototype is a special alternative to $ that will act like\[@%] when given a literal array or hash variable, but will otherwiseforce scalar context on the argument. This is useful for functions whichshould accept either a literal array or an array reference as the argument:

 sub mypush (+@) {
  my $aref = shift;
  die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
  push @$aref, @_;
  }

When using the + prototype, your function must check that the argumentis of an acceptable type.

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

As the last character of a prototype, or just before a semicolon, a @or a %, you can use _ in place of $: if this argument is notprovided, $_ will be used instead.

Note how the last three examples in the table above are treatedspecially by the parser. mygrep() is parsed as a true listoperator, myrand() is parsed as a true unary operator with unaryprecedence the same as rand(), and mytime() is truly withoutarguments, just like time(). That is, if you say

 mytime +2;

you'll get mytime() + 2, not mytime(2), which is how it would be parsedwithout a prototype. If you want to force a unary function to have thesame precedence as a list operator, add ; to the end of the prototype:

 sub mygetprotobynumber($;);
  mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)

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 $_ = $@;
  &$catch;
 }
  }
  sub catch (&) { $_[0] }
 
  try {
 die "phooey";
  } catch {
 /phooey/ and print "unphooey\n";
  };

That prints "unphooey". (Yes, there are still unresolvedissues having to do with visibility of @_. I'm ignoring thatquestion for the moment. (But note that if we make @_ lexicallyscoped, 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;
 }
 @result;
  }

Some folks would prefer full alphanumeric prototypes. Alphanumerics havebeen intentionally left out of prototypes for the express purpose ofsomeday in the future adding named, formal parameters. The currentmechanism's main goal is to let module writers provide better diagnosticsfor module users. Larry feels the notation quite understandable to Perlprogrammers, and that it will not intrude greatly upon the meat of themodule, nor make it harder to read. The line noise is visuallyencapsulated into a small pill that's easy to swallow.

If you try to use an alphanumeric sequence in a prototype you willgenerate an optional warning - "Illegal character in prototype...".Unfortunately earlier versions of Perl allowed the prototype to beused as long as its prefix was a valid prototype. The warning may beupgraded to a fatal error in a future version of Perl once themajority of offending code is fixed.

It's probably best to prototype new functions, not retrofit prototypinginto older ones. That's because you must be especially careful aboutsilent 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 $n\n";
  }

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

 func(@foo);
  func( split /:/ );

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

This is all very powerful, of course, and should be used only in moderationto make the world a better place.

Constant Functions

Functions with a prototype of () are potential candidates forinlining. If the result after optimization and constant foldingis either a constant or a lexically-scoped scalar which has no otherreferences, then it will be used in place of function calls madewithout &. Calls made using & are never inlined. (Seeconstant.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 geta warning by default. (You can use this warning to tell whether or not aparticular subroutine is considered constant.) The warning isconsidered severe enough not to be affected by the -wswitch (or its absence) because previously compiledinvocations of the function will still be using the old value of thefunction. If you need to be able to redefine the subroutine, you need toensure that it isn't inlined, either by dropping the () prototype(which changes calling semantics, so beware) or by thwarting theinlining mechanism in some other way, such as

 sub not_inlined () {
  23 if $];
  }

Overriding Built-in Functions

Many built-in functions may be overridden, though this should be triedonly occasionally and for good reason. Typically this might bedone by a package attempting to emulate missing built-in functionalityon a non-Unix system.

Overriding may be done only by importing the name from a module atcompile time--ordinary predeclaration isn't good enough. However, theuse subs pragma lets you, in effect, predeclare subsvia 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 thebuilt-in name with the special package qualifier CORE::. For example,saying CORE::open() always refers to the built-in open(), evenif the current package has imported some other subroutine called&open() from elsewhere. Even though it looks like a regularfunction call, it isn't: the CORE:: prefix in that case is part of Perl'ssyntax, and works for any keyword, regardless of what is in the COREpackage. Taking a reference to it, that is, \&CORE::open, only worksfor some keywords. See CORE.

Library modules should not in general export built-in names like openor chdir as part of their default @EXPORT list, because these maysneak into someone else's namespace and change the semantics unexpectedly.Instead, if the module adds that name to @EXPORT_OK, then it'spossible 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, quitedeliberately, to the package that requests the import. There is a secondmethod that is sometimes applicable when you wish to override a built-ineverywhere, without regard to namespace boundaries. This is achieved byimporting a sub into the special namespace CORE::GLOBAL::. Here is anexample that quite brazenly replaces the glob operator with somethingthat 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;
 if (opendir my $d, '.') { 
  @got = grep /$pat/, readdir $d; 
  closedir $d;   
 }
 return @got;
  }
  1;

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 (andsubversive) behavior for the glob operator for every namespace,without the complete cognizance or cooperation of the modules that ownthose namespaces. Naturally, this should be done with extreme caution--ifit must be done at all.

The REGlob example above does not implement all the support needed tocleanly override perl's glob operator. The built-in glob hasdifferent behaviors depending on whether it appears in a scalar or listcontext, but our REGlob doesn't. Indeed, many perl built-in have suchcontext sensitive behaviors, and these must be adequately supported bya properly written override. For a fully functional example of overridingglob, study the implementation of File::DosGlob in the standardlibrary.

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

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

The built-ins do, require and glob can also be overridden, but dueto special magic, their original syntax is preserved, and you don't haveto define a prototype for their replacements. (You can't override thedo BLOCK syntax, though).

require has special additional dark magic: if you invoke yourrequire replacement as require Foo::Bar, it will actually receivethe argument "Foo/Bar.pm" in @_. See require.

And, as you'll have noticed from the previous example, if you overrideglob, the <*> glob operator is overridden as well.

In a similar fashion, overriding the readline function also overridesthe equivalent I/O operator <FILEHANDLE>. Also, overridingreadpipe also overrides the operators `` and qx//.

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

Autoloading

If you call a subroutine that is undefined, you would ordinarilyget an immediate, fatal error complaining that the subroutine doesn'texist. (Likewise for subroutines being used as methods, when themethod doesn't exist in any base class of the class's package.)However, if an AUTOLOAD subroutine is defined in the package orpackages used to locate the original subroutine, then thatAUTOLOAD subroutine is called with the arguments that would havebeen passed to the original subroutine. The fully qualified nameof the original subroutine magically appears in the global $AUTOLOADvariable of the same package as the AUTOLOAD routine. The nameis not passed as an ordinary argument because, er, well, justbecause, that's why. (As an exception, a method call to a nonexistentimport or unimport method is just skipped instead. Also, ifthe AUTOLOAD subroutine is an XSUB, there are other ways to retrieve thesubroutine name. See Autoloading with XSUBs in perlguts for details.)

Many AUTOLOAD routines load in a definition for the requestedsubroutine using eval(), then execute that subroutine using a specialform of goto() that erases the stack frame of the AUTOLOAD routinewithout a trace. (See the source to the standard module documentedin AutoLoader, for example.) But an AUTOLOAD routine canalso just emulate the routine and never define it. For example,let's pretend that a function that wasn't defined should just invokesystem with those arguments. All you'd do is:

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

In fact, if you predeclare functions you want to call that way, you don'teven need parentheses:

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

A more complete example of this is the Shell module on CPAN, whichcan treat undefined subroutine calls as calls to external programs.

Mechanisms are available to help modules writers split their modulesinto autoloadable files. See the standard AutoLoader moduledescribed in AutoLoader and in AutoSplit, the standardSelfLoader modules in SelfLoader, and the document on adding Cfunctions to Perl code in perlxs.

Subroutine Attributes

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

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

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

 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 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 codewhich associates them with the subroutine. In particular, the second exampleof valid syntax above currently looks like this in terms of how it'sparsed and invoked:

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

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

Subroutine function

Daftar Isi

NAME

SYNOPSIS