If nothing else, having the initialisation class in the inheritance tree via @ISA, is baggage, if there are no other methods than _init(). Looking back at the OPs code, there seems to be little reason for calling the _init() routines as methods rather than subroutines.

Indeed, as no instance of the initialisation classes is ever instantiated, it could equally be termed "poor OO". And it is a weird use of "inheritance" in every sense. If the idea is simple to provide two methods of initialisation, better I would say to have them both in the main My package space called (say) _initFromXML() and _initFromJSON() and ditch the extra package spaces entirely.

But, as I said, it is quite hard to tell from what the OP posted, as he has obviously simplified the code for the purpose of the question. It is always possible that he also has code in each base package to persist the objects to disk or database, which might make all the difference.