Note that %INC (and likely @INC) is actually a global global. That is, it is available in every package:

package Foo;

sub whence
{
    my $fp = shift;
    $fp =~ s=::=/=g;
    $fp .= '.pm';
    $INC{$fp}
}

package main;

use warnings;

print Foo::whence('warnings'),$/;
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Works just fine. ;-) I make minor changes to your example because I didn't use a "normally named" package to put whence into.