use File::Spec;

    my $mod_path;
    foreach my $i (@INC)
    {
        if ( -f File::Spec->catfile($i, 'Some/Module.pm') )
        {
            $mod_path = $i;
            last;
        }
    }
    if ($mod_path) {
        # Some::Module is installed here.
    } else {
        # Some::Module is not installed anywhere.
    }