%INC maintains a hash of (module filename relative to directories in @INC) => (full path to module file). ie, on my system, I get

$VAR1 = {
          'warnings/register.pm' => '/usr/share/perl/5.8/warnings/regi
+ster.pm',
...
[download]

from use Data::Dumper; print Dumper \%INC

That should tell you where perl is finding your module.