use 5.010;
use strict;
use warnings;

my ($str1, $str2) = ("ABCDE", "BCDEO");

my %in_str1 = map { $_ => 1 } split //, $str1;
my %in_str2 = map { $_ => 1 } split //, $str2;
my @unique;

for my $letter (keys %in_str1, keys %in_str2) {
	unless ( $in_str1{$letter} and $in_str2{$letter} ) {
		push @unique, $letter;
	}
}

say "Unique letters: @unique";