use strict; and use warnings; will help you by identifying mistakes such as declaring a variable twice (in the same scope). The cure is to make sure your variables are declared ONLY ONCE unless there is some special and very good reason for doing otherwise; a situation you're not apt to encounter until you're a very sophisticated and knowledgeable coder.

Always use strict; If you see uncleanliness you can do two things: clean it up or stab out your eyes so you do not see it anymore. Your choice.

So if I want global variables I need to use MAIN::Variable_Name = whatever ok a few questions. Do I have to supply a value when I declare it? Do I have to refer to it as MAIN::Variable_Name or will Variable_Name work? Can I call it within other blocks? And in my full code is there any reason it would fail to open the directory?

Short answers:

1. No.
2. No
3. (Usually) the latter
4. You don't generally "call" a variable, but if you mean can you make use of it, yes
5. Several

This series of questions practically begs for a near-complete Perl tutorial. It seems that's what you need. So please read some documentation; a book; or examples of working code here in the Monastery, rather than jumping to unwarranted conclusions.