Ah, that explains it, my version is 1.50.

I found the problem in that version by running perldoc -m Math::BigFloat, giving the source listing. My pager is less so I'm able to search text by typing "/as_int". That found only the instance in pod. Finding where the as_* methods are by the same method, I noticed as_number and saw from its definition that it was what we wanted.

Since it's been remedied in recent versions, I don't think it's worth bothering the author with it.