#/usr/bin/perl
use strict;
use warnings;
$|++;

require 5.008;
use Hash::Util;

my %hash= (
	one => undef,
	two => 2,
	three => 'abc',
);

Hash::Util::lock_keys( %hash );

print $_,$/ for keys %hash;

## causes runtime error
$hash{add}= 'yes';