Well, if you do an XOR of two strings (scalars that
haven't been used as numbers), perl will do a bitwise
XOR on the characters of the strings, instead of the
bits on the numbers. Two characters that are the same
will have all the bits the same, XORing those characters
will give you the character of which all the bits are 0,
aka "\x00". If two characters are not the same, there is
at least one bitpositions where the bits are different -
resulting in a 1 bit when XORing. Hence, the resulting
character will be anything but "\x00".
It works because the xor of two bits is zero if and only if the two bits are the same:
0 ^ 0 = 0
1 ^ 1 = 0
0 ^ 1 = 1
1 ^ 0 = 1
So the xor of two characters (which are a string of 8 bits) will be 0 (8 0-bits) if and only if the two characters are the same.
Using xor on strings xors the characters from the first string with the characters in the second string, so in the places where the characters are identical the resulting character will be zero, and in all places where they differ it will be nonzero.