#!/usr/bin/perl

use strict;
use warnings;
use Digest::MD5 qw( md5_hex );

while (my $xml = <STDIN>) { 
  $xml =~ s/(\d{11})/md5_hex($1)/eg; 
  print $xml;
}

##</code><code>##

  $xml =~ s/(\<tag\>)(\d{11})(\<\/tag\>)/$1md5_hex($2)$3/eg; 

##</code><code>##

  $xml =~ s/(\<tag\>)(\d{11})(\<\/tag\>)/transformed($1,$2,$3)/eg; 

...
sub transformed {
  my ($prefix,$number,$suffix) = @_
  return $prefix . md5_sum($number) . $suffix;  
}