I'm sorry, but those aren't global variables, they are just lexically scoped variables at the "highest level" of a script.
Declaring variables like this is bad practice, in my mind if you're doing CGI programming, or any other perl programming that creates persistent compiled copies of perl scripts.
This type variable sharing will give the famous "variable will not stay shared" error with mod_perl.
If you're never going to use mod_perl, then you can keep on coding this way. A better way would be to pass all variables to your subroutines as arguments, thusly:
This more rigid, but it really helps you keep your variables in line, making for easier maintaince down the road. You can reuse the subroutine elsewhere, 'cuz you know exactly what goes in and what comes out.
You could declare the variables as globals with the closed perl equivalent, thusly:
but this is bad programming practice generally, as I understand it. (why is this? Is it due strictly to memory usage?)
Update: Of course. I must have had a brain cramp. See TGI's post below.