$ cat hash_lock_keys.pl
#!/usr/bin/perl
use strict;
use warnings;
use Hash::Util qw(lock_keys);

my %f = (alpha=>1, beta=>2, gamma=>3);
lock_keys(%f);

if ($f{apple} == 4) {
	print "Eh?\n";
}

$ perl hash_lock_keys.pl
Attempt to access disallowed key 'apple' in a restricted hash at hash_lock_keys.pl line 9.