This might be sound advice ... in a suitable context ... but you are unwittingly taking for granted that the programmer is doing client-side scripting!

You're absolutely right. I am making an assumption as to the environment (server-side, client-side). That assumption may be incorrect.

I understand that the OP is looking for advice on what language to learn so that they can better understand OOP in general. Classes, interfaces, overridden methods, public/private/static/protected members, inheritance, polymorphism, encapsulation, etc. Yes it's clear that Perl has all of those things (and then some) but what isn't always clear is how Perl's way (if there is such a thing) and the Java/C#/C++ way line up together. The keyword "private" for example is "my" in Perl (sort-of). Those other languages have keywords while Perl has idioms and other tricks to Get The Job Done.

For someone who already understands OOP quite well, Perl's power can be magnified by a factor of magnitude. You could say the same thing for those who have a firm grasp of functional programming. Having previous experience with Perl may help the OP learn traditional OO practices in a language more closely-aligned to Java (such as ActionScript 3) because similar concepts -- though with different spellings -- can be detected in both.

And let the record show that programmers can write really-good or really-awful code in whatever tool they choose to pick up.

Aye. "That...is the sound of inevitability..."

Classes, interfaces, overridden methods, public/private/static/protected members, inheritance, polymorphism, encapsulation, etc.

Only two of those things are important to OOP, and arguably only one is vital.

Only two of those things are important to OOP, and arguably only one is vital.

I suppose you are referring to polymorphism and encapsulation.

Everything else in the list are simply common ways to implement those two aspects of OO. You are absolutely correct.