Looking at the port's dependencies, I found out that it depends on both /dns/p5-Net-LibIDN and dns/p5-Net-LibIDN2 (dragged in as a dependency from other packages).
Removing the dependency on p5-Net-LibIDN and the p5-Net-LibIDN package from the system results in a successful build and the resulting package works fine. Built and tested on 12.0-RELEASE-p10 amd64
Also, grepping spamassassin sources for LibIDN or any of its functions returns no results.
Checked with ports from September 19th.