use strict;
use Hash::Util qw(lock_keys);

my %hash = ();

$hash{Erlaubt}      = 1;
$hash{Test}         = 1;
$hash{Auch_erlaubt} = 1;

lock_keys(%hash);

sub foo {
  my $h = shift;
  $h->{uhu} = 1;
}

foo(\%hash);

#! /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.

use strict;
use Hash::Util qw(lock_keys);

my $hash = { uhu1 => 1 };
lock_keys(%$hash);
$hash->{uhu2} = 0;
^Z
Attempt to access disallowed key 'uhu2' in a restricted hash at - line 6.

sub lock_keys (\%;@)