So, fwiw, I had to do the following to your code to get your method to work the way I wanted and cover all corner cases I've tested. (If the change spans to the very last character for example, then your first condition fails and you never assign $+[0] to $lsubscr ... So I had to do the following.

         .
         .
         .
  ($rold ^ $rnew) =~ /^\0*/ and my $lsubscr = (-$+[0] + 1 && $+[0]) ? 
+-$+[0] + 1 : length $new;
   my $losubscr = ($lsubscr == length $new) ? length $old : $lsubscr;
   substr $old, $losubscr, 0, '</span>';
         .
         .
         .
[download]

This was to fix if the difference was the very last string char and one character is missing from another. Like in the case of data like:

All sentences end with what
All sentences end with what?
[download]

In this case the difference is -1, but because I had to add one to it, it was set to 0. But 0 is the beginning of the string, not the character just past the end of the string.

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