Just one question. Is the Perl interpreter smart enough to know that int($n/$m) should use integer division rather than floating point division?
i.e. would there be any difference in performance betwee int($n/$m) vs. ($n-$r)/$m?
Sorry, you asked to ask two questions. But to save you from having to ask to ask another question, I'll answer it anyway.
Performance should be roughly the same (4 trivial ops in the former, 5 trivial ops in the latter). The latter is better from a numerical methods point of view, no possibility of floating point error.