What is missing:
The new backend is not 100% compatible to the old backend
due to the lack of data sharing. This is a complex issue
but maybe solvable with the help of the mod_perl code.
Alternatively the code could fall back to a single
interpreter when compatibility mode is on. I am trying
to avoid legacy compatibility code in the C part and
move that to a Perl module interposed between back-perl
and the oldstyle module. To make this as easy both for
legacy- and new users is a thing I am still working on.
Personally I dislike the idea of breaking compatibility,
however I don't know how much people actually use the
Perl backend broken as it is. The new API is designed
to be as extendable as possible to avoid incompatible
changes in the future.