Sic Semper NULL talks about how allowing NULLs in a database schema can simplify the schema design. An advantage of using an interpreted language like Perl is that you can have support for undefined values – and we Perl users do, in the form of undef and defined().
Just as in a database, sometimes you just don't know what are the values of your data. undef is perfect for that. 'Nuff said.
