DB<137> $h{$_}++ for (split//,$str1),(split//,$str2)
 => ""

  DB<138> \%h
 => { A => 1, B => 2, C => 2, D => 2, E => 2, O => 1 }

  DB<139> grep {$h{$_}==1} keys %h
 => ("A", "O")