I think that I have to agree with you ;-)

On the other hand, those test cases designed by the author can only be considered as unit test cases. It must then pass UAT (User Acceptance Testing).

Now talk about UAT, the author of the module shall never influence, not to say design, the UAT test cases. But I understand that as an open forum, each tester (I remember they had/have this role for CPAN) only has very limited amount of time to commit, as everybody has their day time job to do. Think about the numbers of CPAN modules people submit all the time, the idea of having formal UAT is also defeated.

Even if there would be a UAT, the quality of the UAT is again a question, as those people who do the UAT for a module may never use it in their real life, and they don't really care (the level they care is much lower than your real users, who do the UAT and will have to live with the bugs they missed ;-).

Now you are end up with doing the UAT yourself. But for well known modules, the situation is quite different, as lots of people used them before you, which means that lots of people have done UAT for you already.