You use %INC just like any other hash, except that its populated automatically by do, require and use. See perlvar for more details.

use strict;
print "$_: $INC{$_}\n" for keys %INC;
__END__
strict.pm: /usr/share/perl/5.10/strict.pm
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Writing to %INC does have its uses. For example, when you want to define classes inside a main script:

use strict;
use warnings;

BEGIN {
    package Foo;
    sub new { bless {}, $_[0] }
    sub ima { "I'm a '", ref $_[0], "' object\n" }
    $INC{'Foo.pm'} = "Anything at all. It really doesn't matter";
}

use Foo;
my $F = Foo->new;
print $F->ima, "\n";
__END__
I'm a 'Foo' object
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This code dies (Can't locate Foo.pm in @INC) without the assignment to %INC. merlyn uses this technique often in his series of articles about Rose::DB.