Great tutorial! I have to say, I am also an avid user of inside-out objects - but I agree with others that it's more an inheritance thing than a typo thing. If it was only about the typo issue you could also use locked hashes (well, erm...) or using array refs with constants as indices or something.

I don't know if you feel like adding it, but there are also some problems with inside-out objects (in general, not with Class::InsideOut, I mean): you have to be more careful about cleanup of the instance data when objects go out of scope, and sometimes it's nice if you can run a hash object through Data::Dumper to debug it.

And the privacy is still relative - I'm not sure how things work under the hood, but wouldn't you be able to step into the package namespace from elsewhere and clobber the hashes holding the instance data?