-a   turns on autosplit mode when used with a -n or -p.   An
          implicit  split  command to the @F array is done as the
          first thing inside the implicit while loop produced  by
          the -n or -p.

               perl -ane 'print pop(@F), "\n";'

          is equivalent to

               while (<>) {
                    @F = split(' ');
                    print pop(@F), "\n";
               }