#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>) {
  my $cmpline = <DATA>; # I always have a pair to compare
  my ($spanold, $spannew) = spandiffstr ($_, $cmpline);
  print "$spanold";
  print "$spannew";
}

sub spandiffstr {
  my ($old, $new) = @_;
  my $span = '<span>'; #eventually I'll pass this in as arg
  my $espan = '</span>'; #ditto
  my @cold = split //, $old;
  my @cnew = split //, $new;
  my $longer = (@cold > @new) ? $#cold : $#cnew;
  my $subscr;
  #find first diff in string from front.
  for (0..$longer) {
     if ($cold[$_] ne $cnew[$_]) {
        $subscr = $_;
        last;
     }
  }
  my $losubscr; #two subscripts because differ len of new and old
  my $lnsubscr; 
  @cold = reverse @cold;
  @cnew = reverse @cnew; #we're gonna search these backwards
  for (0..$longer) { 
    if ($cold[$_] ne $cnew[$_]) {
        $losubscr = @cold - $_;
        $lnsubscr = @cnew - $_;
        last;
    }
  }
  #got pertinent subscripts, now insert span tags
  substr ($old, $losubscr, 0) = $espan; 
  substr ($new, $lnsubscr, 0) = $espan;
  substr ($old, $subscr, 0) = $span;
  substr ($new, $subscr, 0) = $span;
  return ($old, $new); 
}
__DATA__ 
I love Perl.
I love Your Mom.
I love to eat spaghetti.
I love to eat spaghetti and meatballs.
I love to smell the flowers.
I hate to smell the flowers.
Stop! That's enough.
Not really enough.
Blood and Tears are expected.
Love and Marriage are exempted.

##</code><code>##

I love <span>Perl</span>.
I love <span>Your Mom</span>.
I love to eat spaghetti<span></span>.
I love to eat spaghetti<span> and meatballs</span>.
I <span>lov</span>e to smell flowers.
I <span>hat</span>e to smell flowers.
<span>Stop! That's</span> enough.
<span>Not really</span> enough.
<span>Blood and Tears are expec</span>ted.
<span>Love and Marriage are exemp</span>ted.

##</code><code>##

<span>Blood</span> and <span>Te</span>ar<span>s</span> are ex<span>pec</span>ted.
<span>Love</span> and <span>M</span>ar<span>riage</span> are ex<span>emp</span>ted.