side note - I did test this, and it does work if you set INSTALLSITELIB to the directory where the Net::DNS install is (it will also then put the SA Perl modules in the same path).

Setting PERL5LIB will also work, but I'm not sure if you can set PERL5LIB from a .procmailrc - procmail is somewhat careful about what environment variables it lets you set.

spamassassin -D < [file with an email in it]

You'll see:
debug: is Net::DNS::Resolver available? yes
debug: Net::DNS version: 0.48

If you want to be able to look through the output easier, you can do something like:

spamassassin -D < spam 2> spamassassin.out

then grep for "::DNS" in the file spamassassin.out