In many places Perl is called an unmaintainable language. People with brains know that you can write unmaintainable code in any programming language what-so-ever. People who are familiar with Perl know that it can be used to write the uglies code ever, or the most beautiful code ever.
Aside from Perl discussion, here is an excellent document that shows how to write totally unmaintainable code in Java. Most of the rules are generic enough to be applied to any language.
I just hope you will never, for the sake of the humanity, follow any of the rules described.