Actually, I think that Object::InsideOut does manage it, but there is something going on in Set::Scalar that makes the behavior really weird. Set::Scalar does a lot of overloading, and that may be part of the problem. I'm not sure that the problem lies within Object::InsideOut.

Object::InsideOut tries to provide a clean way to drag a "foreign" class into the inheritance tree so that you can freely inherit from non-Object::InsideOut classes. I think I managed to run into a pathological case.

Inside-out objects can, as a general case, be any sort of blessed reference. The key characteristic is that the attributes of the object are not stored in that reference. The ID of the object is not one of the visible attributes, and I think the innards of Object::InsideOut make the referent read-only as well.

Class::InsideOut includes a discussion of what was available on CPAN to support inside-out objects, including brief critical assessments of each, noting strengths and weaknesses. Object::InsideOut fared well, and I don't see this little exercise in digging into packages and experimenting with -MO=concise as contradicting the essence of that.