Don't get confused. They are really like any other scalar in perl, for example:
The one that is the most different is $_ which is simply the default scalar that perl uses for many functions if you do not tell perl which scalar to use, for example:
the above performs chomp on $_.
the above performs chomp on $foo.
(See $_ on the page linked to below.)
Other variables like $/ are just predefined variables that perl uses for many internal operations. Each time you run perl, perl gives them a value (predefined).
They are all discussed on this page and many have examples:
http://perldoc.perl.org/perlvar.html
They are all global in scope (last time I checked). Meaning you have to use the local() function if you want to create a temporary definition for one of them in a block of code. See the local() function documentation.