When you declare a variable, it's like addressing an envelope. (Though I wonder if describing something in terms of envelopes is informative, in these days of email .... Like the other day, I explained soemthing to some younger people in terms of children learning to tie their shoelaces, and found out they grew up with velcro shoes.) When you assign a value, you fill the envelope ... that may or may not require extra space.

Check out PerlGuts Illustrated if you want to know more.

Personally, since I've managed to avoid needing to do anything with the internals, I have found that small doses of guts are best; learning is a cumulative proceess. Another way of looking at it, each time I investigate, I become convinced I don't need to know more.