in reply to Your favourite gory detail...
With regard to the "difference" between \(@array, @brray) and \((@array), @brray)--there is none.
In both cases, the contents of both arrays get flattened into a single list. The backslash is then applied to each element of that list. No amount of extra parens will change the outcome, which is a list of references to the elements of boths arrays.
Update: My (least) favorite gory detail is the limitation of using prototypes to create your own map-like subs that I first encountered in Problem emulating built-ins like map and grep. and still get bitten by from time to time.