use strict;
use warnings;
use Hash::Util 'lock_keys';

my $key = 'foo';

lock_keys my %hash, ($key, 'bar');

$hash{$key}++;

$hash{'key'}++;

__END__

##</code><code>##

$ perl 1230007.pl
Attempt to access disallowed key 'key' in a restricted hash at 1230007.pl line 11.