I guess perrin is talking about Hash::Util, with this, it lets you lock hash keys. One example as how to use that module:

use Data::Dumper;
use Hash::Util;

use strict;
use warnings;

my $constants;
$constants->{CONST_ABXCDEFG} = 1;
$constants->{CONST_ABCXDEFG} = 2;
$constants->{CONST_ABCDXEFG} = 3;
$constants->{CONST_ABCDEXFG} = 4;
$constants->{CONST_ABCDEFXG} = 5;
Hash::Util::lock_keys(%{$constants});

print Dumper($constants);

$constants->{CONST_ABCDEFGX} = 6;
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