creating a temp variable for each entry must have some overhead to worry about

Umm, no, actually creating a temporary variable almost never has enough overhead to worry about. Creating a temporary copy of a large array or hash may impose enough overhead to worry about, but you'd find that out when you find that there is a speed (or other) issue and profile the code. Don't premature optimise code. Write code so it is clear, correct and maintainable first, then optimise if there is a problem.