The () prototype is there to enable inlining the constants, making the code a bit more efficient.

From perlsub:

Functions with a prototype of "()" are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically- scoped scalar which has no other references, then it will be used in place of function calls made without "&". Calls made using "&" are never inlined. (See constant.pm for an easy way to declare most constants.)