Thanks for the compliment, ww. You have a point, that sometimes when everyone comes out with the same suggestion, it reflects cultish thinking. But sometimes it means there really is one best way to do it. When the problem is "find strings from one list in another list," it's just pretty hard to beat a hash lookup for speed and simplicity, and this was a pretty typical case. A hash lookup is so superior to other methods that it makes sense to go to it automatically -- without thinking, even -- unless there's some reason it won't work. It's like using strict: you should always use it unless you know enough to know when not to use it.

On the memory issue, I'm really not sure why the grep-in-a-grep solution ran the OP out of memory. Maybe it causes temporary lists to be built in memory? In any case, a hash isn't all that memory intensive. I created a 10,000 item array, and then turned it into a hash's keys. The hash took 150% as much memory as the array. So edge cases where you have enough memory to use an array but not a hash will be unusual. I agree that solving the problem in less memory could be an interesting challenge, but only worth tackling if a hash lookup fails first.