I wrote a bit about judging CPAN modules here (though the mention of reviews is a bit outdated now). As for "how old is too old", it depends: PayPal's API is likely to have changed many times over the years, so you'd probably want a module that is current. On the other hand, a module that, for example, calculates MD5 sums is likely to still work even if it's 10+ years old. And even in the case where a Perl module isn't available, many modern Web-based APIs are easy enough to access with e.g. HTTP::Tiny and JSON::PP (core modules in modern Perls), or one can do something like I did in Re: Apache Pulsar modules.