As some say, there is only one implementation of the perl interpreter (actually, not true, there's ponie, but that IIUC uses identical parsing). The catch is that undocumented functionality is subject to change, whether purposeful or accidental. The best thing to do when you encounter a case like this where the behavior you see is reasonable is to submit a doc patch and ideally a new test to go with it.

I note that there is an example of my(undef.,..) in perltie back in 5.005_0X.