Iterator Example

        sub make_iterator {

          my @tokens = split /(\#)/, shift();
          my $n_digits = grep {$_ eq '#'} @tokens;
          my $digits = '0' x $n_digits;

          return sub {
              my $result;
              my $d = 0;
              for my $t (@tokens) {
                if ($t eq '#') {
                  $result .= substr($digits, $d++, 1);  
                } else {
                  $result .= $t;
                }
              }

              $digits++;
              if (length $digits > $n_digits) { # Overflow?
                $digits = '0' x $n_digits;      # Reset
              }

              return $result;
            };
        }

Anonymous subroutine is a closure
my variables are caputured by the closure
Each call to make_iterator constructs a new closure with new private state variables