thankyou. My problem is that I am incorporating this in a loop so that I do not know how many Hashes of arrays (ie. how many taxids) I will have each time

here I have tried to illustrate a bit better what I want (although I know its not correct). Or is there some way of pushing or pasting the arrays into a list that I then pass to List::Compare

my %HoAncestors;my @intersection;
foreach my $qu (keys %HoTxHits) {
        my $array_list;my $numb_taxids;
        # go through each taxid, get ancestors, then find the intersec
+tion
        foreach my $taxid (@{$HoTxHits{$qu}}){
                $numb_taxids=scalar(@{$HoTxHits{$qu}});
                my $queryTaxid=$taxid;
                while ((exists $HoPar{$queryTaxid}) && ($HoPar{$queryT
+axid} != 1)) {
                        my $parent=$HoPar{$queryTaxid};
                        push (@{$HoAncestors{$taxid}},$parent);
                        $queryTaxid=$parent;
                }
        }
        my $lcm=List::Compare->new($HoAncestors{$taxid}[0]..$HoAncesto
+rs{$taxid}[$numb_taxids-1]);
        @intersection=$lcm->get_intersection;

}
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