That's correct, there's not a detailed explanation in the Camel book describing that +< (which is based on fopen(3)'s "r+") overwrites rather than inserting. I suppose it's considered beyond the scope of the book to explain filesystem behavior, but it wouldn't hurt for clarification to be included.

perlfaq5 has a FAQ question entitled "How do I change, delete, or insert a line in a file, or append to the beginning of a file?" I think that after reading that you will probably have fewer questions, and a more thorough understanding of how to insert without overwriting.