There are a couple of things going on here. The first is the question of why you should pass variables to a sub instead of using globals, or lexicals that are in the same scope as the sub. The answer is simply that it's easier to see what's going on. The passing of the variable -- and the acceptance of the variable in the sub by reading @_ -- is a kind of documentation, showing what that sub expects. This is standard programming technique stuff. (There's also an issue of creating closures, but that's a little more information than you need right now.)

The other part is whether or not you should pass references. You probably shouldn't when passing scalars, unless they're huge, but all other types are safer and more efficient when passed as references. See the various documentation sources on references for more explanation on that.