After digging through the available resources:
I still found no suitable place to start, because there's not much reference material on formats
There's a lot of discussions on perl5-porters varying from thread implementations to unicode handling and from string interpolation to regular expression irregularities, but format's are not discussed, so the mailing list archives are not a good resource. So I had to do some research myself.
Several mails to the mailing list asking for a place to start the research led to 2 pointers, both proved to be very wrong, because they assumed the flaw was in the formatting code.
Digging. Learning how to use gdb, which - on a HP-UX platform - proved to be less than easy. And though perlguts turned out to be a nice manual for the parts that did not blow up in my face, I had to track down what was happining, and try to follow the flow that is used when parsing formats.
After turning the formatting inside out and upside dow, not finding
anything unusual happening, I just added printf statements
haphazardly throughout the perl source code on points where I did expect
the flow to pass by, and printed out some global variables. At this point
intuition turns out to be more valuable than knowledge, cause I almost hit
the right globals from the start.
Realizing a not very well known fact that the variable declaration within a form definition can span multiple lines if the starting line of those variable start with a open brace, I was intuitively looking for places that dealt with that code.
It turned out to be the closing brace } that caused a
reset of a global brace counter when not aware of being inside a format
parsing. I then did a decrement, instead of a reset, to check if I was
right, and the format part now compiled, but other code parts did not.
So the solution was at hand: the trouble spot was detected, but I had to find a way to detect (or rather know) that at that point I was inside a format and, if so, decrement, otherwise reset.