I'm not sure; a lot of existing plugins use dancer_version, and it's
meaning is fairly clear.

I think the pragmatic approach is to let them continue to do so, with a
couple of tweaks:

 - dancer_version can strip /_\d+$/ so that dev releases don't cause
   issues
 - a special-case for D2 - e.g. return 2 if ($version >= 1.9) - if the
   code is calling dancer_version, it most likely wants to know the
   difference between D1 and D2, I can't see that it's likely to be
   used in many other ways, you'd tend to use $Dancer::VERSION for that.