Crypt::OpenPGP has several things that could be blocking a successful install. One is that it relies directly upon Crypt::IDEA, Crypt::DES, and Crypt::Blowfish, each of which are incompatible with Perl 5.18. If you're using Perl 5.18, I recommend manually installing the "unauthorized release" versions of those modules that I've uploaded. I've been in touch with the author of those modules, and he committed to getting a new release uploaded within 48 hours (that was a couple weeks ago, ;).

The other issue is that Crypt::OpenPGP relies on Crypt::RSA, which depends on Crypt::Random, which has a dependency on Math::Pari. So you'll need to ensure that the PARI/GP library, including dev headers is installed on your system. Some Linux distributions may already have dpkg or aptitude libraries built for PARI/GP, in which case you should be able to install them and get going. I don't know about PARI/GP distributions for Windows though.

Dana Jacobsen created a module called Alt::Crypt::RSA::BigInt which provides an alternate implementation of Crypt::RSA that eliminates the PARI/GP dependency by using Bytes::Random::Secure and Math::BigInt rather than Crypt::Random and Math::Pari. If you install Alt::Crypt::RSA::BigInt, and then try to install Crypt::OpenPGP, you should meet with success. Take heed to the tips in the documentation for Alt::Crypt::RSA::BigInt that will help where performance is an issue.

So to summarize:

• If you're using Perl 5.18, install the "unauthorized release" versions of Crypt::DES, Crypt::IDEA, and Crypt::Blowfish (at least until the author of those modules fixes them).
• If you don't have PARI/GP, install it, and install Math::Pari.
• If you can't install PARI/GP and Math::Pari, install Alt::Crypt::RSA.
• Then re-try installing Crypt::OpenPGP