Yeah - on second thought the distinction isn't as grave as I made it out to be. In fact, I have used that transparent property of inside-out objects myself to "attach" data to foreign objects that didn't like their data structure modified.

If an object already is an inside-out object, you have no choice but to continue down that road. The same holds true for Moose-based (hash) objects.

And in the reverse, if you have a hash-based object, you can glue inside-out extensions to it, and it seems as if you can derive a Moose-based class from it.

If an object already is an inside-out object, you have no choice but to continue down that road.

That's not true.

package InsideOut;

use strict;
use warnings;

use Hash::Util::FieldHash qw[fieldhash];

fieldhash my %name;

sub new {
    my $class = shift;
    bless do {\my $var}, $class;
}

sub set_name {
    my ($self, $name) = @_;
    $name{$self} = $name;
}
 
sub get_name {
    my ($self) = @_;
    $name{$self};
}
 
1;
__END__

package Traditional;

use strict;
use warnings;
use InsideOut;

our @ISA = qw[InsideOut];

sub new {
    my $class = shift;
    bless {}, $class; 
}
 
sub set_colour {
    my ($self, $colour) = @_;
    $self->{colour} = $colour;
}
 
sub get_colour {
    my ($self) = @_;
    $self->{colour};
}
 
1;
__END__

#!/usr/bin/perl

use 5.010;

use strict;
use warnings;

use Traditional;

my $obj = Traditional->new;

$obj->set_name("NAME");
$obj->set_colour("COLOUR");

say "The name is ", $obj->get_name, " and its colour is ", $obj->get_c
+olour;

__END__
The name is NAME and its colour is COLOUR
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As you can see, you can inherit from an inside out object, and use tradition hash based objects in the derived class.

I note that for the simple example there, the methods and variables are completely independent of each other.

What happens when the child needs to affect the parent's values? I suspect it would be difficult to keep the Traditional child pure in a practical situation, and you'd end up with an inflexible hybrid child, or a significant re-implementation of the parent to handle the Traditional hijinks that ensue when get/set methods are skipped.