Thanks for the recommendation davorg ... but you've inadvertently munged the title: Elements of Programming *with* Perl (the link is to the review of the book on this site).

Having written just such a beginner's book, I'll chime in with a few words of advice:

• Don't talk down to the reader.
• Introduce 'strict' and '-w' very early (at the beginning even), and then always use it.
• How we explain something in a classroom environment doesn't often translate well to the printed page -- and we tend to forget all the little extra narrative bits we may have used when we explained concept X. Test out your written material early and often. (this also means writing the book in sequence so you can have a few test students reviewing it as you go rather than waiting until it is complete).
• Definitely emphasize the importance of planning (as you mention).
• Try to keep the book as OS neutral as possible (make it a Perl programming book, not a Win32 or Unix or Mac specific book).
• Get out of the house and do something active -- programming during the day and then writing into the wee hours can take a toll, and depression from just the lack of sunshine can creap up on you.
• Don't always write in the same place, take a notebook (paper variety) to a coffee shop or pub and sketch out some of your ideas and explanations on paper. For one thing, you're out of the house and around other people, and more importantly, it seems a less formal environment than sitting in front of the computer, so your mind may be a little freer to wander around and come up with novel ideas (maybe even ideas for novels :-).
• Above all, enjoy writing it -- writing is hard work (well, it is for me anyway) and if you don't enjoy writing it, it is unlikely that anyone will enjoy reading it.

I wish you the best of luck, and I hope to see your book on the shelves in the future. Every new 'good' book helps prevent the unwary newcomer from wasting their time and money on one of the less than stellar ones that already over-populate the shelves.