Here's a cross-platform version that uses the File::Find core module:

use File::Find;
my @m;
find(sub {if ($File::Find::name =~ m/^\Q$File::Find::topdir\E\/(.+)\.p
+m$/i) { $_ = $1; s/\//::/g; push (@m, $_); }}, @INC);
$, = "\n";
print sort @m;
[download]

It's not very compact but that's mainly because of File::Find's annoying subref way of working. If you don't need the sorting, it can be made quite a bit more compact.