[0] ->  1 4 6 8 9 ...
[1] ->  1 3 5 6 20 ...
[2] ->  2 3 4 5 6 ...
[3] ->  5 7 8 9 12 ...
[4] ->  3 5 8 11 13...
[5] ->  4 5 7 8 9 ...
[6] ->  1 4 5 7 8 ...
...

##</code><code>##

1. 5 -> is shared between all four arrays ([1],[2], [5], [6])
2. 4  -> is shared between 3 arrays ([2],[5], [6])

##</code><code>##

for (1,2,5,6){
   foreach $ar (@{$D[$_]}){
      $hash{$ar}++;
   }
}

# sort %hash
#pick top two