I have read several times "Advanced Perl Programming". That's why I understand how they work but it is not really defined when I have to use them.
I sometimes saw some modules using these technics and I wanted to know if it is recommended to use them when you implement modules/packages.
dragonfly is certainly plenty of wisdom when he says "if you don't know why you should use it, you probably shouldn't be using it."
I didn't ask for a course but just PM to share their own experience on those things. Don't waste too much time on this question.

thank you for your answers.