Sounds very much like homework, so I'll just give you a tip: a hash (e.g. %data) would come in handy. You can use the data (A(01), B(02), etc) as keys and the number of times you encounter the data as values.

  $data{$line}++;
[download]

would do the trick.

As a final step, you iterate over the keys (the function keys is useful here) in the hash and print those keys that have values larger than 1.

Hope this helps, -gjb-