What I'm saying isn't much different than what others have already posted, but why I'm saying it is:

MS Access can be set to accept NULL values. This is your easiest solution.

Being a perl novice as well, I prefer this approach, though I recognize this is not a substitute for proper programming. I am not, however, a newbie to database programming, and while you can make your database accept NULL values, I don't recommend it. If you plan on doing any sort of analysis or statistics of the data you are collecting, having or allowing NULLs may bite you in the rear.

Hope this helps!