From perldoc -f require in 5.7.3:

You can also insert hooks into the import facility, by putting directly Perl code into the @INC array. There are three forms of hooks: subroutine references, array references and blessed objects.

Subroutine references are the simplest case. When the inclusion system walks through @INC and encounters a subroutine, this subroutine gets called with two parameters, the first being a reference to itself, and the second the name of the file to be included (e.g. "Foo/Bar.pm"). The subroutine should return "undef" or a filehandle, from which the file to include will be read. If "undef" is returned, "require" will look at the remaining elements of @INC.

If the hook is an array reference, its first element must be a subroutine reference. This subroutine is called as above, but the first parameter is the array reference. This enables to pass indirectly some arguments to the subroutine. In other words, you can write:

        push @INC, \&my_sub;
        sub my_sub {
            my ($coderef, $filename) = @_;  # $coderef is \&my_sub
            ...
        }
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or:

        push @INC, [ \&my_sub, $x, $y, ... ];
        sub my_sub {
            my ($arrayref, $filename) = @_;
            # Retrieve $x, $y, ...
            my @parameters = @$arrayref[1..$#$arrayref];
            ...
        }
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If the hook is an object, it must provide an INC method, that will be called as above, the first parameter being the object itself. (Note that you must fully qualify the sub's name, as it is always forced into package "main".) Here is a typical code layout:

        # In Foo.pm
        package Foo;
        sub new { ... }
        sub Foo::INC {
            my ($self, $filename) = @_;
            ...
        }

        # In the main program
        push @INC, new Foo(...);
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Note that these hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See "%INC" in perlvar.

--
Ilya Martynov (http://martynov.org/)