Although there are many exceptions, I would say that, most of the time, you don't want the functions of your module to have side effects (such as printing to a file). You want these functions to receive some parameters and to return a result to be used by the caller.
Of course, one obvious exception is if you want to write a logging module, i.e. a module whose aim is precisely to print things out to one or several files. But then, you have to make it generic if you want to re-use it. So either you have to pass back and forth the file handles between the module and the calling program, or you have to store it in such a way that the module will know where to print, which you can do, for example, with closures or with objects.
Feel free to do that for your own training if you wish, but remember that such modules have already been written, thoroughly tested and made available for free, such as Log::Log4perl. That's probably what you should use for solving a real life problem.
