__DATA__
XXXXXXABCDXXXXXXXX
XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX

##</code><code>##

10  characters added
XXXXXXABCD**!!!***!*XX*!!!XXX**XXX

##</code><code>##

use strict;
use warnings;

my $match = 'ABCD';

while (<DATA>)
  {
  my $org = $_;
  defined (my $mutated = <DATA>) or die "Missing edited line";

  my $segment = substr $mutated, 0, index ($mutated, substr $match, 3, 1) + 1;
  my $suffix = substr $mutated, index ($mutated, substr $match, 3, 1) + 1;
  (my $pInsert = $segment) =~ tr/*!//cd;
  (my $pSegment = $segment) =~ tr/*!//d;
  print length ($pInsert) . " characters added\n";
  print "$pSegment$pInsert$suffix\n";
  }

__DATA__
XXXXXXABCDXXXXXXXX
XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX