A search of MetaCPAN for "lock"

This reminds me of the core module Hash::Util:

#!/usr/bin/env perl
use warnings;
use strict;
use Hash::Util qw/lock_hash/;

my %x = ( foo => "bar" );
lock_hash %x;

eval { $x{y}++;       1 } or warn $@;
eval { $x{foo}="quz"; 1 } or warn $@;

__END__

Attempt to access disallowed key 'y' in a restricted hash at lock.pl l
+ine 9.
Modification of a read-only value attempted at lock.pl line 10.
[download]

Although there was some discussion on P5P about removing them a while back, I'm not sure if that will happen.

For completeness, there are also tied hashes, which could implement the same thing.