#!/usr/bin/perl

use strict;
use warnings;

use Digest::MD5 qw(md5_hex);

my @content;

{
  open my $fh, '<', 'null_byte.txt' or die $!;
  local $/ = "\0";
  while ( my $entry = <$fh> ) {
      push @content, $entry, md5_hex( $entry );
  }
  close $fh;
}

open my $fh, '>', 'null_byte_new.txt' or die $!;
print $fh join "\0", @content;
close $fh;