" and I can't see someone saying: "This is the object-system we have settled on and this one should be used because we have decided to so and we include it in some standard" "

But the good news is, whatever object system you would like to use, there probably is already a perl module for it

Anyway, for a beginner the object system he uses is unimportant IMHO. The different object systems are in essence syntactic sugar. What you want to learn are the concepts of oo, not the syntax. So find a good oo book and either go with the system your book uses or make the effort to translate to whatever system you want to use.