I'm interning with a company that is a COBOL shop. It took me months to make the transition from "normal languages". I don't think it's worth learning honestly unless you get a job in it. Here are some things it has taught me.

It's taught me to create pretty code that is indented correctly. I pretty much live in a debugger. Most people out of a CS program think they know how to debug stuff. They don't have a clue. It's taught me to be very meticulous and review every single thing I do down to periods(which BTW terminate loops and if statements making life hell) If you can't understand what you just wrote it needs rewritten.

As far as language comparisons. All variables are declared at the top of the program. All variables are fully global. If you move a variable to a smaller variable it doesn't throw an error it just gets truncated. Loops start at 1 instead of 0. There is very limited error handling.

I don't know of any resources online. I searched when I started working but I didn't find anything real helpful. I did have a 25 year old 30 million line code base to learn from though