It will work for objects which are internally implemented as hashrefs. Moose does this by default, and it's quite a bit of work to persuade it to implement objects any other way.

(There are various MooseX modules on CPAN that can do that work for you, including MooseX::InsideOut and my own MooseX::ArrayRef which allows you to create Moose classes based on arrayrefs.)

It is generally considered poor style to peek at the hashref internals of Moose objects (indeed, blessed objects in general). One good reason is that if you're relying on a module's internals, it gives the module's author less freedom to refactor those internals.

In this case, when you appear to be the author of both modules, so the above might not be a concern, the strong argument against poking the hashref is that it bypasses type constraint checks.

One alternative solution might be to pass a closure from your first class to your second class...

use strict;
use warnings;
use Test::More;

{
    package Thing;
    use Moose;
    has attr => (is => 'rw');
    sub get_closure {
        my $self = shift;
        return sub { $self->attr(@_) };
    }
}

{
    package ThingWithRef;
    use Moose;
    has closure => (is => 'ro');
    sub translate_to_dutch {
        my $self = shift;
        my $str = $self->closure->();
        $str =~ s/e/a/;
        $self->closure->($str);
    }
}

my $obj1 = Thing->new(attr => 'Hello');

my $obj2 = ThingWithRef->new(closure => $obj1->get_closure);
$obj2->translate_to_dutch;

is($obj1->attr, 'Hallo');
done_testing;
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