Thank you for you two answers.

You are both saying : "your program is simple, just do things one after the other, you probably don't need objects." (and giving me examples of how you use TDD, thank you)

Practically I agree, BUT
my question is more theoretical than practical.
I am trying to understand key concepts on a (too) simple example. And i think in a way, i finally understood that my question is "bottom-up or top-down design?". Could anybody confirm this? This is all new vocabulary to me, so i would appreciate if someone experienced told me whether i am understanding the concepts properly or not.

When i will build a bigger, more complex program, i will have the same question again : should i first start building low-level objects (bottom-up approach), thus basing my design on assumptions about external inputs? Or shall i go "outside-in", first interacting with my external peers, and then delegating responsibilities to new lower-level classes when i see that i need to.

The programer who wrote this article prefers a top-down approach , because he doesn't like to base a design on assumptions, and thus write useless code.
While this one prefers bottom-down, because it (would) result in more loosely-coupled elements of reusable code; and it requires a lot less stubs or mocks while in development.

These questions are important to me. Sure, i don't want to become a "purist", and to make my life complicated on simple problems all the time. But i am just trying to learn how to design highly maintainable, loosely-coupled code. And if there are several ways to do it, i'd rather learn about them.

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I don't find it today, but yesterday i read this blog entry by chromatic, saying that he likes to use the smallest possible objects and roles as possible. And i read it in a few other places too. I guess it sounds as if it was "more work", and people will say "why are you being so complicated?" But it also gives maintainability and flexibility. And i don't think that "more classes = more work", as creating classes is so simple with Moose.

PS : thank you for pointing at Test::XT