Kudos! I'm very happy to see more work done in this area, particularly that which addresses some of the glaring shortcomings of Class::Std. I look forward to studying it in more detail.

One point of note -- by storing the id in the blessed scalar, you eliminate the ability of an inside-out object to subclass any kind of object. That is one of the two main benefits of inside-out objects (the other being privacy). (See various threads in Threads and fork and CLONE, oh my!). It's at least worth noting in your documentation as you're using a more specific variation of the inside-out technique that trades speed for some flexibility of design.