Here's another XOR solution that I think is far more to the point: 
sub compare {
  (($_[0] ^ $_[1]) =~ tr/\0//c) < 2;
}

[download]
It counts the number of non-NULLs produced by XORing the strings together. If that number is less than 2, then the strings were either identical or differed by only one character. This could also be written as: 
sub compare {
  ($_[0] ^ $_[1]) =~ /^\0*(?:[^\0]\0*)?$/
}

[download]
The regex ensures the XORed string is either entirely NULLs or has just one non-NULL in it.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: Tell whether two strings differ at only one position by japhy
in thread Tell whether two strings differ at only one position by rg0now

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