for my $thing (grep defined, @oldlist) {
    if (/(\d+-PVC_\d+)/) {
        my $item = $1;
        my $str1 = foo($item) || '';
        my $str2 = bar($item) || '';
        push @newlist, $str1 . '/' . $str2;
    }
    else {
        warn "Do not know what '$thing' is\n";
    }
}