That seems a bit crazy if you ask me. I was programming logo in 5-6th grade and i'll bet that plenty of people here where doing something similar at young ages. I understand not everyone did, but then most people still don't. Making a blanket statment like "Before 7th and 8th grade, children have a lot of trouble with abstract thinking" seems ridiculous to me. I understand you probably didn't mean "all children", but here we start algebra (exponents, variables, equations, etc) in 6th grade and i think many of us certainly could have handled it ealier than that. Hmm seems my rant lost steam somehwere in there. Anyway i think people should have more faith in our childrens ability and desire to learn and cultivate it with projects like this. That teacher who started us on algebra in 6th grade also started me on LOGO and BASIC in 5th grade and had us all going to musicals and reading books that "6th graders wouldn't understand".
Sorry for the off topic rant, to bring it back, i think such a book would be excellent and perl is an excellent language to get your feet wet with. Oh and I hate when people say "xth grader's wouldn't understand that" in case you didn't notice.