Efficiency still is a reason to prefer map->assign over foreach(push). In the case of m// over index, its a case of allowing your needs to grow easily. When I use index I'm making a bet that I will only want to do exact string comparisons. That's frequently not the case over the long run so for that case, the decision often tilts toward m//.

Again, the code isn't written to be instructional to a novice. It is written to be usable by competent perl programmers. I don't much care if a C or VB programmer doesn't understand it right away. That is, assuming the code is in a place that I don't expect novice programmers to be. If I have the expectation that someone with lowered abilities to read perl is going to need to understand it, I will write in a way that maximizes readability. Often that must means using a few more intermediate variables but I know I've switched from a map->assign to foreach(push) just for that context. It is about what is situationally appropriate. I've no hard and fast rule and I don't expect anyone else to either. I do expect that programmers will let the current priorities guide how they work.

I should also note that while I do try to write in a style that prefers runtime efficiency over reabability by novices, I always try to keep it at a modest level and use as many layout hacks as necessary to make the structure apparent to me or someone of equal ability.