2.6 Special Variables
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.