The examples are wrong - you must specify 'Z' at the end of the dates (meaning UTC time):

use Date::Set;

$a = Date::Set->event();
$a->event( rule => 'FREQ=MONTHLY;INTERVAL=2;' );
$a->event( start => '20030801Z',
           end   => '20031201Z' );
@list = $a->list;

foreach (@list) {
  print "$_\n";
}
# output
20030801Z
20031001Z
20031201Z
[download]

I would recommend you to use the DateTime::Event::ICal or DateTime::Event::Recurrence modules instead, because these have a larger user base, and thus have been much better tested.