At first blush, this seems to work. Not sure about the performance; I'll poke at it a bit more. It returns the position of the match, which can be 0, so you have to test if the result is defined. 
sub comp
{
  my($s1,$s2)=@_;
  my @s1 = split('',$s1);
  my @s2 = split('',$s2);
  my $transpose;
  
  for (my $i=0;$i<@s1;$i++)
  {
    if ($s1[$i] ne $s2[$i])
    {
      if ($transpose)
      {
        return undef;
      }
      # Uh-oh; they differ.  Is this a transposition?
      if ( ($s1[$i+1] eq $s2[$i]) &&
           ($s1[$i]   eq $s2[$i+1]))
      {
        $transpose = $i;
        $i++;
      }
    }
  }
  return $transpose;
}

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In reply to Re: Detecting transpositions by sgifford
in thread Detecting transpositions by BrowserUk

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