There are a lot of things we should be able to do without a module, but that's not reality. Modules like Test::Prereq, Test::Pod, Test::Distribution, and so on help us figure out what we forgot to do.

You might be able to track prerequisites without a module, but that tasks gets harder when it's more than a couple people working on the code or the code base gets larger. There's no reason to tell someone that they shouldn't use a computer to check their work. :)

Module::ScanDeps is okay, but it's better to actually run the code and see what it uses rather than parse the code and guess.