thank you for the snippet, I really appreciate it.
I found the need of handling arrays too: append them if found.
to combine {a:[1,2]} and {a:[3,4]} into {a:[1,2,3,4]} I had to edit the while loop in the middle:

   while (my ($k,$v) = each %merged) {                                                      
     my @hashes = grep { ref $_ eq 'HASH' } @$v;
     #$merged{$k} = $v->[0] if (@$v == 1 && !ref $v->[0]);

     # append text separated with \n\n, but do not duplicate identical values
     $merged{$k} = join "\n\n", do { my %seen; grep {!$seen{$_}++} @$v} if (!ref $v->[0]);

     # append arrays one to other
     @{$merged{$k}} = map {@$_} @$v if (ref $v->[0] eq 'ARRAY');

     $merged{$k}  = mergehashes(@hashes) if @hashes;
   }

I thought someone else will find this useful too.