The main disadvantage of not knowing classic Perl OOP is that most of the existing code does not use Moose.

I feel that Moose is massive overkill for having mostly just a (somewhat) nice syntax for declaring your class properties. You also get class introspection where you can introspect the inheritance tree and (Moose-declared) fields of your classes through a defined API. Personally, I haven't seen any tooling that actively exploits this feature, like some enhanced debugger that lets you conveniently watch (Moose) objects, or a webpage that automatically generates forms for Moose objects, but that might be because I haven't looked closely enough or filtered them out by suspecting too much magic at play.

I think that tye's argument against Moose is that it leads to classes being little more than glorified hashes by making the bad things easy (autogenerating accessors) while bringing no support for the good things of Class/Object design, namely designing an API to interact with the object beyond getting/setting properties on the object.

So, what I'm trying to say, from my point of view, only knowing Moose will possibly prevent you from using/implementing a lighter-weight object system. It certainly won't prevent you from writing OOP-style code.