If you've resigned yourself to reinventing the wheel, I suggest taking a look at Net::FTP and using some of the code in that module. Just because you don't feel you can rely on its presence doesn't mean you can't use some of the same code.

Additionally, check out the CPAN module for ways of programmatically determining what modules are installed and what versions they are. Note that CPAN probably requires some LWP:: or Net:: modules, so that may be out as well.