I'm probably just dumb (okay, probably not probably), but it took me way too long to understand the difference between package variable and lexical variables.

An article that helped me greatly is one by Dominus called Coping with Scoping. Like I said, I found it very useful (as is often the case with Dominus' articles 'n stuff). Check it out if the above answers still leave you wondering, or even if they don't.

good luck