What I might better have written was that mismatching substrings do not have to be the same length in the strings being compared. Consider the approach of Re: Fastest way to find the mismatch character with the input strings:

my $varA = 'This is the wild cat';
my $varB = 'Thsi is the house cat';
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The naive bitwise approach used does not allow for 're-synchronization' after mismatching substrings of differing length, so the output does not seem to be what is desired:

Th<mismatch>si</mismatch> is the <mismatch>house cat</mismatch>
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(Of course, the OPer may not need this sort of feature at all, in which case: No problem!)