Net::DNS is required by some services, including SpamAssassin.
It's best to install it using cpan, eg:
cpan -i Net::DNS
Sometimes you may want to try it manually for , and can do so like this:
tar xvzf Net-DNS-1.20.tar.gz
keeping in mind, that if you're doing it manually because cpan was failing some tests, although you might just be bypassing the tests, you're not bypassing the errors, so you may actually run into run-time issues with SpamAssassin. Sometimes the tests are for cases that might not happen often, so you could be fine to do it this way.