Thus the mechanism is: Equip each object (magically) with one hidden hash, its corona. Use that hash, keying by class name, to store the individual alter ego of the object for each class.
This reminds me of another "make inheritance work" technique I'd heard of. Every object is a hash ref (as usual), but every class has their own key into it. Basically every object looks like the $corona in clinton's description. The Alter advantage over that method is mainly privacy. The various classes sharing space can't meddle with each other's attributes. If ego is changed to address tye's concern (so that a class can specify its own name), I'm guessing that Alter loses that advantage too. At that point, you might as well write a base class with ego in it.