If you wonder about my definition of FALSE, see Truth and Falsehood in perlsyn:
From the pod you are reffering to:
The number 0, the strings '0' and '' , the empty list () , and undef are all false in a boolean context. All other values are true. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as '' , but as a number, it is treated as 0.
I still believe it's more customary to use undef as the ultimate null and false value in Perl. For example in DBI, DBIx::Class and alike, if you set a field to undef it will store in the database as an actual NULL whereas if you use '' (the empty string) you will get a blank string in the database, probably not what you'd want. I have a feeling that the explicit returning of undef is a quite popular convention :)
This "ultimate null/false discussion" is probably steering OT and should move to a new thread IMO, though I would suspect that it has been discussed ad nauseam here in PM.