If you know how to document, test, and debug a complete script, you have a much greater chance of succeeding at one-liners. For the first step, you should learn your shell's peculiarities, the way that Unix passes command-line arguments to Perl and Perl's interpretation of those arguments. In Unix you'll see the concept of an executable task, a process that is usually a program loaded into memory. Processes are started by other processes, except for the init process, which is usually started by the kernel, and sometimes kernel processes. As far as the user is concerned, starting a process requires a shell or a launcher.
Processes get command-line arguments.
But unlike C, Perl steals some of those arguments from the script for its own purposes. The shell separates arguments on space characters. See the perldoc perlrun page for further information on the switches that Perl has to offer from the command line. Perhaps some examples would be best at this point. In the spirit of this column, let's use one-liners.