##
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")
##
##
DB<146> grep {$h{$_}==1} map {$h{$_}++;$_} split//,$str1.$str2
=> ("A", "O")