#!/usr/bin/perl -w 

use strict;
use warnings;
my $string1="abcdef";
my $string2="defghi";

my @ary=split(//,$string1);
push @ary,split(//,$string2);
my %een=();

map{ $een{$_}++ } @ary;
my @unique=();
foreach (@ary){
	next unless $een{$_} == 1;
	push @unique,$_;
}

printf "%s\n",join(",",@unique);