They aren't.

But solid Ruby programmers use them this way and audit third-party code before using it and avoid it if it contains such code smell.

One of the fun things about Ruby is that you can open up, override and chain just about anything.

Apart from allowing for some interesting experiments, it also allows you to do things like monkey-patch faulty methods in libraries (including the std libs) at run-time while you wait for a sanctioned update to come out.

This may or may not be a Good Thing, depending on who you are and what you're doing.

I don't think global variables were mentioned in the article. Do you mean the global (top-level) namespace? If so, then yeah, "Don't do that" is pretty much the solution.

Sorry, maybe this isn't correct Ruby nomenclature, but I see Array simply as a global object, which can be modified, with rather obvious consequences. It's sad that I feel old enough to say this, but isn't this a problem that we solved a long time ago?

"Don't do that" is one solution, Alan Kay's idea of "worlds" is another.