I suggest you look at using some of the Pure Perl crypt modules. This way you will not have to fork or pipe to an external program.
I have successfully used the pure perl Crypt::OpenPGP
with POE on windows. If you use this module, it will handle the compression of your plaintext before encryption as well.
If you like rolling your own, like we all do, here are some quick links to cpan. You may have a little work to do finding these when it comes time to install on windows, due to ActiveState's selective ppm distributions and encryption export regulations, but if you're this far along then you already know that :-)
Crypt::CBC The encryption and decryption process is about a tenth the speed of the equivalent SSLeay programs (compiled C). This could be improved by implementing this module in C. It may also be worthwhile to optimize the DES and IDEA block algorithms further.