Has this always been so?

Indeed it has. If you look at the top of the warn and die docs you'll see the prototype func LIST meaning it takes a list. Although this isn't terrifically relevant to the output as the list is concatenated into a single string and the same behaviour goes for the arguments to the __WARN__ and __DIE__ signal handlers. So it's really just for your convenience, such is perl's way :)
HTH

_________
broquaint