$ perl -le'

my $varA = "This is the wild cat";
my $varB = "Thsi is the widl cat";

my $temp = $varA ^ $varB;
my @fields;
unshift @fields, [ $+[0], $-[0] ] while $temp =~ /[^\0]+/g;

my $VarC = $varB;
for my $offsets ( @fields ) {
    substr $VarC, $offsets->[0], 0, "</mismatch>";
    substr $VarC, $offsets->[1], 0, "<mismatch>";
    }
print $VarC;
'
Th<mismatch>si</mismatch> is the wi<mismatch>dl</mismatch> cat