$a = ord('A'); # 01000001 $b = ord('B'); # 01000010 $a = $a ^ $b; # 01000001 XOR 01000010 = 00000011 $b = $a ^ $b; # 00000011 XOR 01000010 = 01000001 $a = $a ^ $b; # 00000011 XOR 01000001 = 01000010