This seemed like a perfectly reasonable question. I gave it an upvote which resulted in: "Reputation: 0". So, someone had downvoted your post. Why? Because you had the temerity to question dogma? I wasn't impressed with this but there's little I can do about it.

As with many things, there's a spectrum with many shades of grey between black and white at the extremities. It is rare for either "black" or "white" to be optimal; a compromise somewhere in the "grey" is usually the best option. This applies equally to software development: writing all of the code first, then bolting on tests afterwards, is a bad move; similarly, writing all tests first, which will obviously fail until the code is written afterwards, is also a bad move; what is needed is a compromise.

What follows is how I achieve this compromise. I'm not suggesting this is in any way perfect; it is, however, something to consider in terms of the principles involved. Probably the main point is that the "black" and "white" extremes are avoided.

I start most modules with module-starter and use Module::Starter::PBP as a plugin. I like the templating facilities provided by Module::Starter::PBP but not the templates themselves (so I've edited those quite substantially). I have many versions of the configuration which vary depending on: personal code, $work code, Perl version, type of module, and so on — the following refers to personal code for v5.36.

This gives me a directory structure along the lines described above. The module code looks like this:

package Some::Module;

use v5.36;

our $VERSION = '0.001';

1;

__END__

=encoding utf8

... POD templates and boilerplate ...
[download]

The t/ directory will contain equivalents of the three 99-*.t Author Only scripts shown above, and a template for 00-load.t which looks like:

#!perl

use v5.36;

use Test::More tests => 1;

BEGIN { use_ok('__MODULE__') }

diag "Testing __MODULE__ $__MODULE__::VERSION";
[download]

Applying a global s/__MODULE__/Some::Module/ to that file gives me a working distribution. I can now run the well-known incantation:

perl Makefile.PL
make
make test
[download]

I have created application and test code in unison: the compromise.

From here, the specifics will vary with every module; however, the main principle is to add small amounts of functionality and concomitant tests incrementally. Continue doing this until all functionality is coded and has tests.

In closing, I'll just note that the OP's title had "What to test"; I've added "[When to test]" to my title indicating this subthread is straying from the original. We actually don't know if Bod had already written all of his tests except the one he asked about, or if he was adding tests as an afterthought. Assuming the latter, and rebuking him for it, was a mistake in my opinion.