You have already stated that the problem only occurs when Apache is involved.

Have you stopped and restarted Apache?

Does the problem persist when the Perl script is run as a CGI program instead of being run as a mod_perl script?

Check the versions of all modules loaded into mod_perl and compare them between the two machines:

for my $module (sort keys %INC) {
    (my $namespace = $module) =~ s!/!::!g;
    $namespace =~ s/\.pm$//;
    print sprintf "Module: %s; Path: %s; Size: %d; VERSION: %s\n",
        $module, $INC{$module}, -s($INC{$module}), $namespace->VERSION
+;
}
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Note that you need to run the above output from within Apache, because when run from the shell everything seems to be OK for you.

Thanks! It was a great idea to compare the %INC of the two virtual servers. I did, and unfortunately they are identical. %INC in both has about 350 keys, and the outputs including version numbers are identical.

And yes, I restarted apache several times while debugging this.

Back to the original problem: my understanding is that decode_json('{"a":true}')returns a ref to a hash with key 'a' and value true which isa JSON:PP:Boolean. If there a way to print attributes of decode_json('{"a":true}')->{a} such that I can compare them between the two servers?

%INC is supposed to contain paths :)

Running on both virtual server apache servers,

    my $json = '{"a":true}';
    $d = decode_json($json);
    printf "Dumper: %s<br>\n",Dumper($d);
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prints the same: Dumper: $VAR1 = { 'a' => bless( do{\(my $o = 1)}, 'JSON::PP::Boolean' ) };

Yet encode_json($d) fails on one and works correctly on the other.

In checking what I thought were reasonable assumptions, I found that JSON::PP's handling of booleans and JSON::XS's handling of booleans are simply incompatible.

use JSON::XS();
use JSON::PP();
JSON::XS->new()->encode( JSON::PP->new()->decode('[true]') );
__END__
encountered object '1', but neither allow_blessed nor convert_blessed 
+settings are enabled
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Note that even enabling convert_blessed() won't help as JSON::PP generates booleans that don't even have a TO_JSON() method. [And enabling allow_blessed() would just turn true/false into null.]

Both modules could use some work on dealing with booleans.

- tye