++ed by:

157 PAUSE users
132 non-PAUSE users.

Gurusamy Sarathy


perlsub - Perl subroutines


To declare subroutines:

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

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

To define an anonymous subroutine at runtime:

    $subref = sub BLOCK;            # no proto
    $subref = sub (PROTO) BLOCK;    # with proto

To import subroutines:

    use PACKAGE qw(NAME1 NAME2 NAME3);

To call subroutines:

    NAME(LIST);    # & is optional with parentheses.
    NAME LIST;     # Parentheses optional if predeclared/imported.


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 even generated on the fly using eval or anonymous subroutines (closures). You can even call a function indirectly using a variable containing its name or a CODE reference to it.


Any arguments passed to the routine come in as the array @_. Thus 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 if a reference to it is taken. (Some earlier versions of Perl created the element whether or not it was assigned to.) Note that assigning to the whole array @_ removes the aliasing, and does not update any arguments.

The return value of the subroutine is the value of the last expression evaluated. Alternatively, 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 will return an empty list in a list context, an undefined value in a scalar context, or nothing in a void context. If you return one or more arrays and/or hashes, these will be flattened together into one large indistinguishable list.

Perl does not have named formal parameters, but in practice all you do is assign to a my() list of these. Any variables you use in the function that aren't declared private are global variables. For the 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;


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

    sub get_line {
        $thisline = $lookahead;  # GLOBAL VARIABLES!!
                $thisline .= $lookahead;
            else {
                last LINE;

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


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

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

    sub upcase_in {




    sub upcase {
        return unless defined wantarray;  # void context, do nothing
        my @parms = @_;
        return wantarray ? @parms : $parms[0];

Notice how this (unprototyped) function doesn't care whether it was passed real scalars or arrays. Perl will see everything as one big long flat @_ parameter list. This is one of the ways 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);

Do not, however, be tempted to do this:


Because like its flat incoming parameter list, the return list is also flat. So all you have managed to do here is stored everything in @a and made @b an empty list. See "Pass by Reference" for alternatives.

A subroutine may be called using the "&" prefix. The "&" is optional in modern Perls, and so are the parentheses if the subroutine has been predeclared. (Note, however, that the "&" is NOT optional when you're 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. See perlref for more on that.)


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

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


Not only does the "&" form make the argument list optional, but it also disables any prototype checking on the 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 the section on Prototypes below.

Function whose names are in all upper case are reserved to the Perl core, just as are modules whose names are in all lower case. A function in all capitals is a loosely-held convention meaning it will be called indirectly by the run-time system itself. Functions that do special, pre-defined things BEGIN, END, AUTOLOAD, and DESTROY--plus all the functions mentioned in perltie. The 5.005 release adds INIT to this list.

Private Variables via my()


    my $foo;            # declare $foo lexically local

A "my" declares the listed variables to be confined (lexically) 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 builtins like $/ must currently be localized with "local" instead.

Unlike dynamic variables created by the "local" operator, lexical variables declared with "my" are totally hidden from the outside world, including any called subroutines (even 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 the 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 the file scope.

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

(An eval(), however, can see the 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.)


    $arg = "fred";        # "global" variable
    $n = cube_root(27);
 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 the variables in its argument list, the "my" doesn't change whether those variables are viewed as a scalar or an array. So

    my @FOO = <STDIN>;


    my $foo = <STDIN>;


    my $foo, $bar = 1;                  # WRONG

That has the same effect as

    my $foo;
    $bar = 1;


    my $x = $x;

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

    my $x = 123 and $x == 123


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

        $line = lc $line;
    } continue {
        print $line;


    } else {
        chomp $answer;

the scope of $answer extends from its declaration throughout the rest of the conditional (including elsif and else clauses, if any), but not beyond it.


The foreach loop defaults to scoping its index variable dynamically (in the manner of local; see below). However, if the index variable is prefixed with the keyword "my", then it is lexically scoped 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, and so the value of $i is unavailable in some_function().

Some users may wish to encourage the use of lexically scoped variables. As an aid to catching implicit references to package variables, if you say

    use strict 'vars';

then any variable reference from there to the end of the enclosing block must either refer to a lexical variable, or 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 principle usefulness of this is to quiet use strict 'vars'. The actual initialization is delayed until run time, so it gets executed appropriately; every time through a loop, for example.


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


    package main;
    local $x = 10;
    my    $x = 20;

That will print out 20 and 10.

You may declare "my" variables at the outermost scope of a file to hide any such identifiers totally from the outside world. This is similar to C's static variables at the file level. To do this with a subroutine requires the use of a closure (anonymous function with lexical access). If a block (such as an eval(), function, or package) wants 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 };



Peristent Private Variables




        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 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 sub around it to make sure it gets executed before your program starts to run:

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

See perlrun about the BEGIN function.

If declared at the outermost scope, the file scope, then lexicals work someone like C's file statics. They are available to all functions in that same file declared below them, but are inaccessible from outside of the file. This is sometimes used in modules to create private variables for the whole module.

Temporary Values via local()

NOTE: In general, you should be using "my" instead of "local", because it's faster and safer. Exceptions to this include the global punctuation variables, filehandles and formats, and direct manipulation of the Perl symbol table itself. Format variables often use "local" though, as do other variables whose current value must be visible to called subroutines.


    local $foo;                 # declare $foo dynamically local

                                #     @merlyn is really @randal, etc

A local() modifies its listed variables to be "local" to the enclosing block, eval, or do FILE--and to any 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.


    for $i ( 0 .. 9 ) {
        $digits{$i} = $i;
    # assume this function uses global %digits hash

    # now temporarily add to %digits hash
    if ($base12) {
        # (NOTE: not claiming this is efficient!)
        parse_num();  # parse_num gets this new %digits!
    # old %digits restored here



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


    local $foo = <STDIN>;

supplies a scalar context.



    tie %ahash, 'APackage';
       local %ahash;
       tie %ahash, 'BPackage';
          local %ahash;


        local %ENV;
        tie %ENV, 'MyOwnEnv';
    [..normal %ENV behavior here..]


    @ary  = ( 0..5 );
         local($ary[5]) = 6;
             print "$e . . .\n";
             last unless $e > 3;
         if (@ary) {
             delete $hash{'a'};

Perl will print

    6 . . .
    4 . . .
    3 . . .
    This is a test only a test.

Passing Symbol Table Entries (typeglobs)



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;



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 my.



        local $/ = undef;
        local $_ = <>;  

It particular, its important to localize $_ in any routine that assigns to it. Look out for implicit assignments in while conditionals.


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

    sub ioqueue {
        return (*READER, *WRITER);
    ($head, $tail) = ioqueue();



        grow();                         # really calls shrink()
    grow();                             # get the real grow() again




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


Pass by Reference


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


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


    sub inter {
        foreach $href (@_) {
            while ( $k = each %$href ) {


Where people get into trouble is here:




    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:

    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 (well, and local()s) are in the symbol table.


    sub splutter {
        my $fh = shift;

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

Another way to do this is using *HANDLE{IO}, see perlref for usage and caveats.


    sub openit {
        my $name = shift;
        local *FH;

Although that will actually produce a small memory leak. See the bottom of "open()" in perlfunc for a somewhat cleaner way using the IO::Handle package.


As of the 5.002 release of perl, if you declare

    sub mypush (\@@)

then mypush() takes arguments exactly like push() does. The declaration of the function to be called must be visible at compile time. The prototype affects only the interpretation of new-style calls to the function, where new-style is defined as not using the & character. In other words, if you call it like a builtin 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}.

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

Because the intent is primarily to let you define subroutines that work like builtin commands, here are the prototypes for some other functions that parse almost exactly like the corresponding builtins.

    Declared as                 Called as

    sub mylink ($$)             mylink $old, $new
    sub mypop (\@)              mypop @array
    sub mykeys (\%)             mykeys %{$hashref}
    sub myopen (*;$)            myopen HANDLE, $name
    sub mypipe (**)             mypipe READHANDLE, WRITEHANDLE
    sub myrand ($)              myrand 42
    sub mytime ()               mytime

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

Unbackslashed prototype characters have special meanings. Any unbackslashed @ or % eats all the rest of the 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 * does whatever it has to do to turn the argument into a reference to a symbol table entry.

A semicolon separates mandatory arguments from optional arguments. (It is redundant before @ or %.)


    mytime +2;

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

The interesting thing about & is that you can generate new syntax with it:

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

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

That prints "unphooey". (Yes, there are still unresolved issues having to do with the 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 grep:

    sub mygrep (&@) {
        my $code = shift;
        my @result;
        foreach $_ (@_) {



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


    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, the func() now gets passed in 1, that is, the number of elements in @foo. And the split() gets called in a scalar context and starts scribbling on your @_ parameter list.


Constant Functions

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

The following functions would all be inlined:

                                                # 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 BAZ_VAL () {
        if (OPT_BAZ) {
            return 23;
        else {
            return 42;

    sub N () { int(BAZ_VAL) / 3 }
    BEGIN {
        my $prod = 1;
        sub N_FACTORIAL () { $prod }

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 the calling semantics, so beware) or by thwarting the inlining mechanism in some other way, such as

    sub not_inlined () {
        23 if $];

Overriding Builtin Functions

Many builtin 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 builtin functionality on a non-Unix system.

Overriding may be done only by importing the name from a module--ordinary predeclaration isn't good enough. However, the subs pragma (compiler directive) lets you, in effect, predeclare subs via the import syntax, and these names may then override the builtin ones:

    chdir $somewhere;
    sub chdir { ... }

To unambiguously refer to the builtin form, one may precede the builtin name with the special package qualifier CORE::. For example, saying CORE::open() will always refer to the builtin open(), even if the current package has imported some other subroutine called &open() from elsewhere.

Library modules should not in general export builtin 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 the name to the @EXPORT_OK list, 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 the overrides.

The foregoing mechanism for overriding builtins 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 builtin 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;

    sub glob {
        my $pat = shift;
        my @got;

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

    package Foo;


The REGlob example above does not implement all the support needed to cleanly override perl's glob operator. The builtin glob has different behaviors depending on whether it appears in a scalar or list context, but our REGlob doesn't. Indeed, many perl builtins 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.


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 package.) If, however, there is an AUTOLOAD subroutine defined in the package or packages that were searched for 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 $AUTOLOAD variable in the same package as the AUTOLOAD routine. The name is not passed as an ordinary argument because, er, well, just because, that's why...

Most AUTOLOAD routines will load in a definition for the subroutine in question using eval, and then execute that subroutine using a special form of "goto" that erases the stack frame of the AUTOLOAD routine without a trace. (See the standard AutoLoader module, 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 call system() with those arguments. All you'd do is this:

    sub AUTOLOAD {
        my $program = $AUTOLOAD;
        system($program, @_);
    who('am', 'i');


    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 Unix programs.

Mechanisms are available for modules writers to help split the modules up 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 perlxs.


See perlref for more about references and closures. See perlxs if you'd like to learn about calling C subroutines from perl. See perlmod to learn about bundling up your functions in separate files.