From the subject line and your first question, I would think you are trying to reserve space for your file, which would be an OS dependant function, and not supported on the systems I am used to use, though I have a distant memory of VSAM where you have to do so.

However, your second question makes me think - do you want a maximum size for your file, or automatically splitting at a given size - perhaps IO::File::Cycle does what you need?