The latest build here was done with -DDEBUGGING (which required some amount of debugging to get to build :-). I had to fix up perlsfio.h in 5.6.1 to be, basically, what is in 5.7. Apparently PerlIO_set_ptrcnt relied on FILE_cnt instead of PerlIO_getcnt (or whatever), where FILE_cnt is specifically only defined if these PerlIO_* macros are not ... wheee!
I got more extensions built, most notably Devel::DProf, ByteLoader, and B. Errno still hasn't built (I am not sure how to get it built), Devel::DProf needs some work, and B won't build out of the box (just some makefile stuff).
B passes all tests after some portability fixes (except for one test, which relies on $Config{static_extensions} or whatever having the right value, which it currently doesn't). Very cool.
So all in all, a lot done today. Hopefully Matthias can answer a few questions for me, and I will load up some patches (including some tests) tomorrow to send on to p5p.