This behavior does not surprise me too much, since undef can be thought of as “a value is not there at all.”

In other words, undef is not the same as “null” or “empty string.” It's not like a null-value (e.g. in a database), where a definitive value has been provided even though that value is “the absence of a value.” Rather, this is indistinguishable from the case where “the question has never been answered, period.” What is a session-data-store supposed to do with something that “does not exist?” A software-designer's quandary, at best... and the designer's choice here is quite defensible.

Perhaps you can be more specific with your question? That's quite a lot of code to wade through, when it's hard to see just what you're getting at... Can you provide a little more guidance to the Monks who are anxious to help you?