You shouldn't force them one way or the other. Make the dependency optional; detect the presence of Log4perl at run time; use if present.

Personally, I'd isolate the logging bits behind a generic interface, that is, a wrapper; then I'd make another wrapper for a different logging module. If it turns out to be easy, you could do several. Detect the presence of any of them at run time (in some kind of priority order), and use one if found. I've done something like this for XML parsing modules.