Ack! I overlooked the unsorted elements in your data. By way of apology, please accept this working, tested code. It is somewhat under-documented and needs refactoring, but I am out of time tonight. For your two sample @sets, my code outputs:

    [ 0, undef, 1, undef, undef, 2, 3, 4, 5, 6 ],
    [ 0, 1, 2, 3, 4, 5, 6, undef, 7, 8 ],
    [ 0, undef, 1, 2, 3, 4, 5, undef, 6, 7 ],
[download]

    [ 0, 1, 2, undef, undef, 3, 4 ],
    [ 0, 1, undef, 2, undef, 3, 4 ],
    [ 0, 1, undef, undef, 2, 3, 4 ],
[download]

The code works by transforming all the lists into AoAs, with each element holding [$original_item, $original_index]. It merges the first two lists, via Algorithm::Diff (++belg4mit) to form elements [$original_item, $index_from_list1, $index_from_list2]. List 3 is then merged in, etc. I *think* it will work fine for *any* number of lists.

Yowza! This looks cool, and I'll check it out.

---

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

Re: your "XXX" block, I've refactored the code thus:

        # 0 --> identical element
        if ( $which_list == 0 ) {
            die "Can't happen" if @indices1 != @indices2;
            foreach my $i ( 0 .. $#indices1 ) {
                my ( $item1, @offsets ) = @{ $list1_aref->[ $indices1[
+$i] ] };
                my ( $item2, $offset  ) = @{ $list2_aref->[ $indices2[
+$i] ] };
                die unless $item1 eq $item2;
                push @offsets, $offset;
                push @merged, [ $item1, @offsets ];
            }
        }

        # 1 --> list 1 has an element here
        if ( $which_list & 1 ) {
            foreach ( @{ $list1_aref }[ @indices1 ] ) {
                my ( $item1, @offsets ) = @{ $_ };
                push @offsets, undef;
                push @merged, [ $item1, @offsets ];
            }
        }

        # 2 --> list 2 has an element here
        if ( $which_list & 2 ) {
            foreach ( @{ $list2_aref }[ @indices2 ] ) {
                my ( $item2, $offset ) = @{ $_ };
                my @offsets = map { undef() } 1 .. $offset_width;
                push @offsets, $offset;
                push @merged, [ $item2, @offsets ];
            }
        }
[download]

I've scrapped the 'elsifs' and used & instead of ==.

---

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

Yes, I could, but that would be solving a different problem. The out-of-orderness is something I have to take into account here.

At least I can be sure the values themselves are unique, but they're not sortable.

---

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart