while ($date <=$end_date) {
.
.
.
(manipulate data here)
.
.
.
($this_year, $this_month, $this_day) = ($date =~/(\d{4})(\d(2})(\d{2})/)
if (!checkdate ($this_year, $this_month, $this_day) {
  # a new month dawns
  $this_day=1; 
  if ($this_month ==12) {#did december just pass
    $this_year++;      
    $this_month=0;
  }
  $this_month++;  #add one to the month.
  $date = $this_year . $this_month . $this_day;
}
}