#!/usr/bin/perl
use warnings;
use strict;

{   package MyRole;
    use Moo::Role;
    has +x => (is => 'ro');
}

{   package MyObj;
    use Moo;
    has x => (is => 'rw');

    sub finalize {
        'Moo::Role'->apply_roles_to_object($_[0], 'MyRole');
    }
}

my $o = 'MyObj'->new;
$o->x(12);

$o->finalize;
$o->x(13);  # x is a read-only accessor