Using defined offsets into arrays/lists, to my mind, has little to offer over the normal positional arg passing

As I understand the idea in POE, one reason for it is forward compatibility. You can rearrange the order of positional arguments -- e.g. insert new ones between fixed arguments and a variable list of arguments at the end -- and since the constants are redefined at the same time, code using the defined offsets still just works.

IMO, PBP should be taken with (several) grains of salt. It's only one opinion about best practices and even the author says it's not an exclusive list of best practices. And there are some stunningly bad recommendations in there, too.