Continuing with what Corion said (read the documentation), if you have a module installed on your system, you can access its documentation by using perldoc from your command line. For instance, to read the docs on File::Find::Rule, you would type:

perldoc File::Find::Rule

Also, to learn how to use perldoc you can type perldoc perldoc and to get a listing of all of the documentation for perl (not modules you may have on your system), type perldoc perl