Also related is TDD because writing a test first forces you to focus on module interface early and from the point of view of the module user.

In your example discussion, module tests that employed a mock DNS server running on a different port would have caught this specific module interface issue before the module was released. Not only that, having your own mock DNS server controlled by your tests makes it easier to test module robustness and recovery (for example, how your module handles timeouts, broken connections, data corruption, ...).

And a test would be pretty much impossible to run with the current version, unless the test was run as root AND unless there wasn't already a process running at the default port.

perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'

Yes indeed! An example of having to resort to mind-boggling trickery and deviousness simply to modify the default value of a module parameter! :) While entertaining, and a challenging problem, definitely not an example of good module design.

From On Interfaces and APIs, the most relevant point is probably this simple tip for Perl module designers from TheDamian:

• Make common usage the default; allow uncommon usage via optional attributes.