stupid question, why do you all put "my" in front of the scalars?

Not stupid at all - it is to limit the scope. This is a very important concept in computing generally and is not just limited to Perl. However, you could do an awful lot worse than read the canonical article by Dominus: Coping with Scoping which deals with the whole issue of variable declaration in Perl. It goes into more depth that you'll need just now but it won't steer you wrong.