It is worth noting here that there are some variables that are considered "special" by Perl. These variables are usually either read-only variables that Perl sets for you automatically based on what you are doing in the program, or variables you can set to control the behavior of how Perl performs certain operations.
Use of special variables can be problematic, and can often cause unwanted side effects. It is a good idea to limit your use of these special variables until you are completely comfortable with them and what they do. Of course, like anything in Perl, you can get used to some special variables and not others, and use only those with which you are comfortable.