Generally, your questions aren't about HTML::TreeBuilder so much as they are about basic perl. So, the first question is: what is your technical background? Are you experienced with other programming languages (HTML is not a programming language, it's a mark-up language, though JavaScript/EMCAScript is a programming language)? If so, which one(s)? The reason this is helpful in helping you is so that we know how much depth is needed in describing the concepts, or if there are parallels between Perl and any language(s) you already know which may help solidify the difference in syntax.

Conversely, if you don't know any languages, then we can describe it differently, and point out different resources that are more targetted to the new programmer.

From your question, I can't tell how much background you might have in programming, so I'm afraid that if I started to explain anything, it might either gloss over stuff and confuse you more, or it could be talking down to you about things you already know. Either way, perlintro might be helpful, though, so I'd suggest starting there.