I cant come up with a good example right now, but there are definitely times where i'd want false (but defined) to indicate a behavior different than for undef.

Likely, shemp was thinking along the lines of an optional numeric option. A crazy example I can think, goes along the lines of "how many errors will this parser tolerate before throwing an exception".

You could leave the option undef to mean "no amount of errors will make me throw an exception", zero (also false) to mean "allow no errors at all" and a positive value (true) to mean "allow for up to this many errors".

However, the most common usage is to have undef stand for "not specified/wanted" and a true value for the contrary.