What you are doing is basically a factory design pattern
where the factory (in your case the My:: package) usually is not part of the hierarchie of the classes it creates.
I mean, it should be natural to get an object of class My instead of My::XML or My::JSON.
I am not sure if I understand your question, but clients of your factory would usually not bother weather they have a XML or a JSON instance but work through the interface that the base-class (My::Base) provides.
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