Be very careful doing this and make sure you're really sure what exactly is "unused" (via something like Devel::Cover or the like (as was mentioned above) and ample tests). Due to the dynamic nature of perl it's entirely plausible to have "unreachable" code that still could be called in that one corner case that you didn't test (e.g. that one strange routine that gets chosen from a dispatch table only every fourth blue moon during months not containing an R when run by a user named rufus).

(And make sure you're using good source control practices so you can back out to the last working version when you remove too much. :)