>perl -wMstrict -le
"for my $ar_vector (
   [ qw(ABCDE BCDEO) ], [ qw(ABCDE BBBBB) ], [ qw(ABCDEA BCDEO) ],
   ) {
   my ($s1, $s2) = @$ar_vector;
   printf qq{'$s1' '$s2' -> '%s' \n}, non_common($s1, $s2);
   }
 ;;
 sub non_common {
   my ($sa, $sb) = @_;
   ;;
   (my $a_sans_b = $sa) =~ s{ [\Q$sb\E]+ }{}xmsg;
   (my $b_sans_a = $sb) =~ s{ [\Q$sa\E]+ }{}xmsg;
   return $a_sans_b . $b_sans_a;
   }
"
'ABCDE' 'BCDEO' -> 'AO'
'ABCDE' 'BBBBB' -> 'ACDE'
'ABCDEA' 'BCDEO' -> 'AAO'