Yes, you can write it with @copy and returning it as \@copy. But I prefer using $copy, for symmetrical reasons. I start out with a reference to an array, so if I copy it, I'd like to copy to another reference. But that's just personal preference, I don't one way is significantly faster or easier to understand than the other.

As for copying using a map instead of the double loop is mostly because it's less verbose. It's just one line and I see immediately it's a copy of a 2D array - the double loop takes more lines, and too large to immediately see what it does. I haven't benchmarked it, but if there's a difference I expect the difference to be small, and the map to be slightly faster; with the difference slowly growing if the arrays get larger. But speed isn't my main reason to copy it using a map.