#! /usr/bin/perl

use strict;
use warnings;
use Data::Dumper;
use Hash::Util qw(lock_keys);

my %hash = (test => {key => 1, kes => 2},
            test2 => {key => 1, kes => 2},
           );

lock_keys(%{$hash{test2}});

$hash{test}->{uhu} = 1;
print Dumper(\%hash);

$hash{test2}->{uhu} = 1;
print Dumper(\%hash);

perl lock.pl
$VAR1 = {
          'test2' => {
                       'kes' => 2,
                       'key' => 1
                     },
          'test' => {
                      'uhu' => 1,
                      'kes' => 2,
                      'key' => 1
                    }
        };
Attempt to access disallowed key 'uhu' in a restricted hash at lock.pl line 17.