use constant DEBUG => 0;
  if (DEBUG) {
    warn(
      "This code is only compiled into the program ",
      " when DEBUG is true.\n";
    );
  }

##</code><code>##

  macro num_max (x,y) {
    ((x) > (y) ? (x) : (y))
  }

##</code><code>##

  print "You owe: \$", {% num_max $total-$paid, 0 %}, "\n";

##</code><code>##

  unless ($expression) {  # include
    ... lines of code ...
  } elsif ($expression) { # include
    ... lines of code ...
  } else {                # include
    ... lines of code ...
  }                       # include