I use Test::MockObject::Extends version 0.20 and Params::Validate version 0.76, both of them come with debian stable.

Anyway, I have found a workaround. I have changed the test script like this:

#!/usr/bin/perl

use strict;
use warnings;

use Test::MockObject;

my $mock = Test::MockObject->new;
$mock->fake_module(
    'MyApp::Property',
    new => sub { bless {}, 'MyApp::Property' },
    id => sub { 1 },
    su_pwd => sub { 'password' }
);

my $mock_property = MyApp::Property->new;

my $admin = MyApp::Admin->new(owner => $mock_property, id => 0);
is_deeply(
    $admin, 
    {
        admin_id => 0,
        domain_id => '1',
        firstname => 'Super',
        lastname => 'User',
        username => 'su',
        password => 'password',
    },
    "new() with id eq '0' (superuser)"
);
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