I found that i could use the --plugin=Module::Starter::Smart argument, to use the plugin. Now it works properly.
I still don't know why the configuration line's plugin: Module::Starter::Smart didn't work previously. While all the others worked properly.
Edit: Well, i get it now. I can use one plugin or the other, but not both, as they inherit from the same class (Module::Starter::Simple). So, Module::Starter::PBP and Module::Starter::Smart won't work simultaneously.
Well... They will, if i cheat. I just changed Module/Starter/Smart.pm, so that it inherits from Module::Starter::PBP, instead of Module::Starter::Simple. It works !
Is there any way to make a cleaner solution, from this dirty one?