Here's another version that uses String::DiffLine. It's about 3 times faster than my previous post, according to Benchmark.

sub comp2
{
  return undef if (length($_[0]) != length($_[1]));
  my $i = diffline($_[0],$_[1]);
  if ( ($i < length($_[0])) &&
       (substr($_[0],$i+1,1) eq substr($_[1],$i,1)) &&
       (substr($_[0],$i,1) eq substr($_[1],$i+1,1)) &&
       (substr($_[0],$i+2) eq substr($_[1],$i+2)) )
  {
    return $i;
  }
  return undef;
}
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String::DiffLine needs some tweaking to compile (change sv_undef to PL_sv_undef in the .xs file), but it seems to be quite fast.