Because there's no pressing need to define it? Perl has historically been a try-it-and-see language, with the actual details of how different features interact left up to the implementation. There was at one point a counter-movement picking some areas and trying to make them well defined, resulting in doc such as perlnumber, but it never really took off.

So, it boils down to "try it and see". And if you can imagine a different implementation doing something differently, be prepared for a different implementation doing it differently :).