See the core module Hash::Util.

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

my $key = 'foo';

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

$hash{$key}++;

$hash{'key'}++;

__END__
[download]

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

Hope this helps!