I was using the very latest JSON, but not the very latest JSON::PP, so that's a factor. Upgrading JSON::PP and specifically using that has fixed it.

It doesn't require JSON::PP or JSON::XS : it works perfectly well with JSON newer than v4.0:

#!perl -l

use 5.012; # strict, //
use warnings;
use JSON 4.0;
use Data::Dumper;

select STDERR;

print $JSON::VERSION;

my $json_text = q({"flag":true});
my $json = JSON->new;
my $json_perl = $json->decode($json_text);
print "Before boolean_values = ", Dumper($json_perl);
$json->boolean_values(0,1);
my $json_perl_after = $json->decode($json_text);
print "After boolean_values = ", Dumper($json_perl_after);

__END__

4.02
Before boolean_values = $VAR1 = {
          'flag' => bless( do{\(my $o = 1)}, 'JSON::PP::Boolean' )
        };

After boolean_values = $VAR1 = {
          'flag' => 1
        };
[download]

Unlike what you said in Re^5: How do I make the JSON module convert true/false to 1/0 instead of a blessed Boolean object?, I believe the documentation is quite explicit about version requirements for that method: the text "boolean_values (since version 4.0)" is in bold header text in the POD, which I interpret as claiming that the method has only been supported since version 4.0. The only clarification in that section of the docs that I would like: not using brackets for "optional" arguments in POD, especially in code blocks, because brackets have very well-defined meaning in Perl, and boolean_values(0,1) works whereas boolean_values([0,1]) as their example implies does not.

(Note also that your example code in Re^2: How do I make the JSON module convert true/false to 1/0 instead of a blessed Boolean object? swapped the order of the arguments, so a false would decode as 1 and a true as 0, which would cause you some difficulty at some point.)

JSON.pm isn't a parser. It's just a front-end for JSON::PP or JSON::XS. So one most definitely needs JSON::PP or JSON::XS.

Maybe you meant one doesn't need to use JSON::PP directly, but the parent post was clear that JSON.pm was the module being used directly.

Since you're installing modules anyway, why not install JSON::XS. Doing so will make JSON.pm a "million" times faster.

Caveat: It doesn't support threads.

Note: I don't use JSON.pm because it defaults to the slow JSON::PP. I used to use JSON::XS directly.

Note: I don't use JSON::XS because of the author's attitude and his unwillingness to support threads and who knows what else. I use Cpanel::JSON::XS directly.

JSON.pm supports Cpanel::JSON::XS, but requires the use of an env var. That said, it doesn't support boolean_values.