#!/usr/bin/env perl
use warnings;
use strict;
use Hash::Util qw/lock_hash/;

my %x = ( foo => "bar" );
lock_hash %x;

eval { $x{y}++;       1 } or warn $@;
eval { $x{foo}="quz"; 1 } or warn $@;

__END__

Attempt to access disallowed key 'y' in a restricted hash at lock.pl line 9.
Modification of a read-only value attempted at lock.pl line 10.