First, you'll just end up confusing yourself again if you call a variable used to hold a datastructure "$json". Also, printing to a file is just adding unnecessary complexity that makes it harder to test. Just print, and see the output. Here's a small script with those adaptations:

#!/usr/bin/env perl

use strict;
use warnings;
use JSON::PP;

my $data = {
        'book' => {
            'title' => JSON::PP::true,
            'pages' => 5.03 #this number
        }
    };

my $json = encode_json($data);
print $json, "\n";

__END__
{"book":{"pages":5.03,"title":true}}
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The part after __END__ is the output I saw on my screen. It indicates that you are indeed getting 5.03, the number. Are you seeing different results? If so, it probably has to do with the heuristics that JSON::PP uses to decide if something is a number or a string. It might do some introspection to determine if the number has ever been used as a string. Examples of what can make Perl think internally that a scalar variable holds a string include obvious string operations such as concatenation, less obvious such as sprintf, and even the common case of using print.

Your actual code may be something like pages => $number, but you may at one point have treated $number as a string. And in so doing, Perl starts thinking the variable holds a string (this is an internal process... an implementation detail leaking out). You can force numification by using a numerical operator at a carefully chosen time:

my $data = {
    book => {
        title => JSON::PP::true,
        pages => 0+$number,
    }
};
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Now we're forcing numeric context on $number.

my $json = {
    'book' => {
        'title' => JSON::PP::true,
        'pages' => 5.03
    }
};

print $json->{'book'}{'pages'}, "\n";
my $string = encode_json($json);
print "After printing: $string\n";

$json->{'book'}{'pages'} += 0;
my $string2 = encode_json($json);
print "After numify: $string2\n";
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5.03
After printing: {"book":{"title":true,"pages":"5.03"}}
After numify: {"book":{"title":true,"pages":5.03}}
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Upgrade? I get this (with JSON::PP 2.27203)–

Yes, I used bignum and it caused this error. Before this code block I added "no bignum" (couldn`t remove it, because it used before this block of code)

use bignum; changes 5.03 to Math::BigFloat->new('5.03').

It`s rather strange, however, I run this script for perl 5.10 - and it worked there,but for perl 5.20 - no, this error occures.

I'm getting a very similar error...

my $payload = {
    'sender'        => {
            'name'  => $vars{'fromname'},
            'email' => $vars{'frommail'},
        },
    'to'            => [{
            'name'  => $vars{'name'},
            'email' => $vars{'email'},
        }],
    'subject'       => $vars{'subject'},
    'HTMLcontent'   => $body,
};
$payload->{'textContent'} = $vars{'preview'} if $vars{'preview'};

my $attrs = {
    'headers'   => $headers,
    'content'   => encode_json $payload,
};
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This is part of the email module we use to send emails through Brevo. But I'm seeing an error...

encountered object 'Site::Email=HASH(0x55fa0a139c18)', but neither all
+ow_blessed, convert_blessed nor allow_tags settings are enabled (or T
+O_JSON/FREEZE method missing)
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The error line is encode_json.

The weird part is that nearly identical code is running elsewhere on the same server without a problem! It's been suggested that bignum could affect it but we are not using that module in either place.

I'm aware of this bug report but that wouldn't explain why one place fails and another doesn't.

Dumpling $payload gives the result one would expect - any ideas on how I further debug this issue?

This is Perl v5.36 on Debian 12

UPDATE...

The problem turned out to be the way I was calling the module...and using the same variable twice!

my $email = Site::Email->new;
my $resp  = $email->send(
    'fname'     => $fname,
    'user'      => "$prefix$data{'idCRM'}",
    'token'     => $token,
    'template'  => 'newuser',
    'name'      => $to,
    'email'     => $email,
    'subject'   => 'Your Book Boost Account...'
);
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