#!/usr/bin/env perl

use strict;
use warnings;
use JSON;
use Scalar::Util qw/ readonly /;

my $data = decode_json('{ "val":false }');
print "readonly: ", readonly($data->{val}) ? 'yes' : 'no', "\n";
bless $data->{val};

##</code><code>##

$./try
readonly: no
Modification of a read-only value attempted at ./try line 10.