And may be I should write it first, and then use it

There's nothing wrong with that approach, of course. On the other hand, object->JSON conversion isn't all that difficult, so I added it to my example (I also fixed a typo where I had user_name instead of username).

In my example, the Moose class definition plays the role of the DOM, so you don't need to store class metadata in JSON. JSON does not know about classes, so you need to tell the JSON encoder what to do if it hits an object. To do this, I use the convert_blessed property of the JSON::PP encoder and define a TO_JSON method which just reduces an object to its underlying hash reference.

A final note before we get to the code: I've been using JSON::PP because it's in Perl core. The code works just as fine with JSON which will use one of the faster XS implementations if they're available.

use 5.028;
use warnings;

package MyCoolRestObject;
use Moose;
use Moose::Util::TypeConstraints;
with 'MyCoolRestObject::Serializer';

has body_raw     => ( is => 'ro' );
has url          => ( is => 'ro' );
has datetime     => ( is => 'ro' );
has entry_id     => ( is => 'ro' );
has subject_html => ( is => 'ro' );
coerce 'MyCoolRestObject::Poster' =>
    from 'HashRef' => via { MyCoolRestObject::Poster->new(%$_) };
has poster       => ( is => 'ro',
                      isa => 'MyCoolRestObject::Poster',
                      coerce => 1,
);
coerce 'MyCoolRestObject::Icon' =>
    from 'HashRef' => via { MyCoolRestObject::Icon->new(%$_) };
has icon         => ( is => 'ro',
                      isa => 'MyCoolRestObject::Icon',
                      coerce => 1,
);
has subject_raw  => ( is => 'ro' );
has body_html    => ( is => 'ro' );

# --------------------------------------------------------------------

package MyCoolRestObject::Icon;
use Moose;
with 'MyCoolRestObject::Serializer';

has url          => ( is => 'ro' );
has comment      => ( is => 'ro' );
has picid        => ( is => 'ro' );
has keywords     => ( is => 'ro' );
has username     => ( is => 'ro' );

# --------------------------------------------------------------------

package MyCoolRestObject::Poster;
use Moose;
with 'MyCoolRestObject::Serializer';

has display_name => ( is => 'ro' );
has username     => ( is => 'ro' );

# --------------------------------------------------------------------

package MyCoolRestObject::Serializer;
use Moose::Role;

sub TO_JSON { +{ shift->%* } }

# --------------------------------------------------------------------

package main;
use Cpanel::JSON::XS;
$/ = undef;
my $json = decode_json <DATA>;

# Cpanel::JSON::XS -> Object:
my $obj = MyCoolRestObject->new(%$json);
say $obj->icon->username;

# Object->Cpanel::JSON::XS:
my $encoder = Cpanel::JSON::XS->new->utf8->pretty->convert_blessed;
my $json_out = $encoder->encode($obj);
say $json_out;

__DATA__
{
    "body_raw" : "This is <b>Entry 1</b>",
    "url" : "http://nataraj.nataraj.hack.dreamwidth.net/459.html",
    "datetime" : "2022-08-20 16:36:00",
    "entry_id" : 459,
    "subject_html" : "Entry 1",
    "poster" : {
        "display_name" : "nataraj",
        "username" : "nataraj"
    },
    "icon" : {
        "url" : "http://www.nataraj.hack.dreamwidth.net/userpic/1/3",
        "comment" : "4",
        "picid" : 1,
        "keywords" : [
            "3"
        ],
        "username" : "nataraj"
    },
    "subject_raw" : "Entry 1",
    "body_html" : "This is <b>Entry 1</b>"
}
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