AutoLoader - load subroutines only on demand

        package Foo;
        use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine

        package Bar;
        use AutoLoader;              # don't import AUTOLOAD, define our own
        sub AUTOLOAD {
            $AutoLoader::AUTOLOAD = "...";
            goto &AutoLoader::AUTOLOAD;

    The AutoLoader module works with the AutoSplit module and the "__END__"
    token to defer the loading of some subroutines until they are used
    rather than loading them all at once.

    To use AutoLoader, the author of a module has to place the definitions
    of subroutines to be autoloaded after an "__END__" token. (See
    perldata.) The AutoSplit module can then be run manually to extract the
    definitions into individual files auto/

    AutoLoader implements an AUTOLOAD subroutine. When an undefined
    subroutine in is called in a client module of AutoLoader, AutoLoader's
    AUTOLOAD subroutine attempts to locate the subroutine in a file with a
    name related to the location of the file from which the client module
    was read. As an example, if is located in
    /usr/local/lib/perl5/, AutoLoader will look for perl subroutines
    POSIX in /usr/local/lib/perl5/auto/POSIX/*.al, where the ".al" file has
    the same name as the subroutine, sans package. If such a file exists,
    AUTOLOAD will read and evaluate it, thus (presumably) defining the
    needed subroutine. AUTOLOAD will then "goto" the newly defined

    Once this process completes for a given function, it is defined, so
    future calls to the subroutine will bypass the AUTOLOAD mechanism.

  Subroutine Stubs
    In order for object method lookup and/or prototype checking to operate
    correctly even when methods have not yet been defined it is necessary to
    "forward declare" each subroutine (as in "sub NAME;"). See "SYNOPSIS" in
    perlsub. Such forward declaration creates "subroutine stubs", which are
    place holders with no code.

    The AutoSplit and AutoLoader modules automate the creation of forward
    declarations. The AutoSplit module creates an 'index' file containing
    forward declarations of all the AutoSplit subroutines. When the
    AutoLoader module is 'use'd it loads these declarations into its callers

    Because of this mechanism it is important that AutoLoader is always
    "use"d and not "require"d.

  Using AutoLoader's AUTOLOAD Subroutine
    In order to use AutoLoader's AUTOLOAD subroutine you *must* explicitly
    import it:

        use AutoLoader 'AUTOLOAD';

  Overriding AutoLoader's AUTOLOAD Subroutine
    Some modules, mainly extensions, provide their own AUTOLOAD subroutines.
    They typically need to check for some special cases (such as constants)
    and then fallback to AutoLoader's AUTOLOAD for the rest.

    Such modules should *not* import AutoLoader's AUTOLOAD subroutine.
    Instead, they should define their own AUTOLOAD subroutines along these

        use AutoLoader;
        use Carp;

        sub AUTOLOAD {
            my $sub = $AUTOLOAD;
            (my $constname = $sub) =~ s/.*:://;
            my $val = constant($constname, @_ ? $_[0] : 0);
            if ($! != 0) {
                if ($! =~ /Invalid/ || $!{EINVAL}) {
                    $AutoLoader::AUTOLOAD = $sub;
                    goto &AutoLoader::AUTOLOAD;
                else {
                    croak "Your vendor has not defined constant $constname";
            *$sub = sub { $val }; # same as: eval "sub $sub { $val }";
            goto &$sub;

    If any module's own AUTOLOAD subroutine has no need to fallback to the
    AutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplit
    subroutines), then that module should not use AutoLoader at all.

  Package Lexicals
    Package lexicals declared with "my" in the main block of a package using
    AutoLoader will not be visible to auto-loaded subroutines, due to the
    fact that the given scope ends at the "__END__" marker. A module using
    such variables as package globals will not work properly under the

    The "vars" pragma (see "vars" in perlmod) may be used in such situations
    as an alternative to explicitly qualifying all globals with the package
    namespace. Variables pre-declared with this pragma will be visible to
    any autoloaded routines (but will not be invisible outside the package,

  Not Using AutoLoader
    You can stop using AutoLoader by simply

            no AutoLoader;

  AutoLoader vs. SelfLoader
    The AutoLoader is similar in purpose to SelfLoader: both delay the
    loading of subroutines.

    SelfLoader uses the "__DATA__" marker rather than "__END__". While this
    avoids the use of a hierarchy of disk files and the associated
    open/close for each routine loaded, SelfLoader suffers a startup speed
    disadvantage in the one-time parsing of the lines after "__DATA__",
    after which routines are cached. SelfLoader can also handle multiple
    packages in a file.

    AutoLoader only reads code as it is requested, and in many cases should
    be faster, but requires a mechanism like AutoSplit be used to create the
    individual files. ExtUtils::MakeMaker will invoke AutoSplit
    automatically if AutoLoader is used in a module source file.

    AutoLoaders prior to Perl 5.002 had a slightly different interface. Any
    old modules which use AutoLoader should be changed to the new calling
    style. Typically this just means changing a require to a use, adding the
    explicit 'AUTOLOAD' import if needed, and removing AutoLoader from @ISA.

    On systems with restrictions on file name length, the file corresponding
    to a subroutine may have a shorter name that the routine itself. This
    can lead to conflicting file names. The *AutoSplit* package warns of
    these potential conflicts when used to split a module.

    AutoLoader may fail to find the autosplit files (or even find the wrong
    ones) in cases where @INC contains relative paths, and the program does

    SelfLoader - an autoloader that doesn't use external files.

    "AutoLoader" is maintained by the perl5-porters. Please direct any
    questions to the canonical mailing list. Anything that is applicable to
    the CPAN release can be sent to its maintainer, though.

    Author and Maintainer: The Perl5-Porters <>

    Maintainer of the CPAN release: Steffen Mueller <>

    This package has been part of the perl core since the first release of
    perl5. It has been released separately to CPAN so older installations
    can benefit from bug fixes.

    This package has the same copyright and license as the perl core:

                 Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999,
            2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2011
            by Larry Wall and others
                                All rights reserved.
        This program is free software; you can redistribute it and/or modify
        it under the terms of either:
            a) the GNU General Public License as published by the Free
            Software Foundation; either version 1, or (at your option) any
            later version, or
            b) the "Artistic License" which comes with this Kit.
        This program is distributed in the hope that it will be useful,
        but WITHOUT ANY WARRANTY; without even the implied warranty of
        the GNU General Public License or the Artistic License for more details.
        You should have received a copy of the Artistic License with this
        Kit, in the file named "Artistic".  If not, I'll be glad to provide one.
        You should also have received a copy of the GNU General Public License
        along with this program in the file named "Copying". If not, write to the 
        Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
        MA 02110-1301, USA or visit their web page on the internet at
        For those of you that choose to use the GNU General Public License,
        my interpretation of the GNU General Public License is that no Perl
        script falls under the terms of the GPL unless you explicitly put
        said script under the terms of the GPL yourself.  Furthermore, any
        object code linked with perl does not automatically fall under the
        terms of the GPL, provided such object code only adds definitions
        of subroutines and variables, and does not otherwise impair the
        resulting interpreter from executing any standard Perl script.  I
        consider linking in C subroutines in this manner to be the moral
        equivalent of defining subroutines in the Perl language itself.  You
        may sell such an object file as proprietary provided that you provide
        or offer to provide the Perl source, as specified by the GNU General
        Public License.  (This is merely an alternate way of specifying input
        to the program.)  You may also sell a binary produced by the dumping of
        a running Perl script that belongs to you, provided that you provide or
        offer to provide the Perl source as specified by the GPL.  (The
        fact that a Perl interpreter and your code are in the same binary file
        is, in this case, a form of mere aggregation.)  This is my interpretation
        of the GPL.  If you still have concerns or difficulties understanding
        my intent, feel free to contact me.  Of course, the Artistic License
        spells all this out for your protection, so you may prefer to use that.