my @list= "external";
s[(?<=<)([^>]+)(?=>))]{
    my $tag= $1;
    if(  $tag eq "item"  ) {
        "$list[-1]-$tag";
    } else {
        if(  $tag eq "end-list"  ) {
            pop @list   if  1 < @list;
        } elsif(  $tag =~ /(\w+)-list/  ) {
            push @list, $1;
        }
        $tag;
    }
}ge;