sub do_something {
      my $a = shift; # assign 1st arg to $a
      my $b = shift; # assign 2nd arg to $b
      my $c = shift; # assign 3rd arg to $c

      ### now do something with $a, $b, $c

      return ($a, $b, $c);
   }

   my $x = 1;
   my $y = 2;
   my $z = 3;
   my ($x, $y, $z) = do_something($x, $y, $z);

Having a subroutine
   1. take parameters in, and assign to local variables
   2. process the local variables
   3. return what needs to be returned