There's no need to have any particular structure. The many "$foo is better than perl" articles already on the web don't follow any particular structure.
In the announcement I've assumed that people either know what INTERCAL, Befunge and Brainfuck are, or will quickly find a description via a search engine such as Google.
It's probably best to assume that the reader knows the general idea of both the language (and of perl for that matter), but not that much on any specific syntax constructions that you want to describe or compare. The idea isn't to be a tutorial on either language, but to parody articles that try to demolish one language to make another look better. They don't usually spend time giving an introduction to either language they refer to.
If you have the URL of one or more good sites that describe the language you can refer to it in your article, or we can add it as a link in the navigation next to the section that links to your article.
There isn't a definite answer for this. A draft I've seen was 500 words, which felt quite short before I read it, but it made many good funny points in an effective writing style. An shorter article that efficiently puts across ideas will be funnier than a longer article which has no more ideas, just padding. And funnier is more important than space filling.
If you submit something that the judges think is too short (or too long and verbose) then they are free to send it back to you with suggestions on how they think it could be improved. This isn't supposed to be a very formal competition, and the role of the judges is to get the funniest articles, not to determine who is the funniest author.