Inserting a Null date.

I am trying to add entries to my database via a Perl script, where some
fields are optional, that is, nulls are allowed. When inserting an
entry, I include every field in the insert statement, but leave their
value undefined if it is optional and left blank. This works for every
field but date fields. Setting a date field to undefined generates the
error "Bad date external representation". Is there a way to leave a date
field null while still having it as an argument in an insert (i.e. INSET
INTO table (id, mydate) VALUES (1, NULL) ), or will I need to make
several different insertion statements for each different case?

Thank you,
Rob Mosher