There does look like one possibility. Perlguts mentions that with Perl 5.16 and earlier: Copy-on-write (see the next section) shared a flag bit with read-only scalars. So the only way to test whether sv_setsv , etc., will raise a "Modification of a read-only value" error in those versions is: SvREADONLY(sv) && !SvIsCOW(sv)

Scalar::Util doesn't do anything special for 5.16 or earlier, so it could have false positives in that case. Of course, 5.18 (where it's fixed) was released in 2013 :)

What do you make of this?

#!/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};
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That produces this for me (on Perl 5.18.2):

$./try
readonly: no
Modification of a read-only value attempted at ./try line 10.
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cant reproduce that here, try upgrading perl and or json module

Or cite yours…?

readonly: no
Modification of a read-only value attempted at - line 9.
# Successfully installed JSON-4.02 (upgraded from 2.97001)
readonly: no
Modification of a read-only value attempted at - line 8.
# Scalar::Util is up to date. (1.50)
perl -MScalar::Util=readonly -MJSON -E 'say readonly(JSON::false) ? "r
+eadonly" : "ReAdOnLY"'
ReAdOnLY
# This is perl 5, version 18, subversion 4 (v5.18.4)
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