Hello kardon,
The best place to learn about this is to just read perldoc.
perldoc defined
I don't know the specifics of what this is aiming to accomplish. However, a literal translation is if the subroutine _ARTNET_H_ is undefined, define a new subroutine OpPoll() to the value 0x2000. Why they are wanting to do this, I can't say. But this is the type of thing that you often see in modules that are attempting to work in more than work environment. Say Mac, Windows, Unix, etc.
- Miller