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);