my $date_string = sprintf('%02d%02d%02d', $year-1900, $month, $day);

##</code><code>##

use Time::Local qw( timegm );

my @start_date = ( $start_year, $start_month, $start_day );
my @end_date   = ( $end_year,   $end_month,   $end_day   );


my $start_date = timegm( 0,0,0, $start_day, $start_month-1, $start_year );
my $end_date   = timegm( 0,0,0, $end_day,   $end_month-1,   $end_year   );

for (my $date=$start_date; $date<=$end_date; $date+=24*60*60) {
   my ($year, $month, $day) = (gmtime($date))[5,4,3];
   $year  += 1900;
   $month += 1;

   my $date_string = sprintf('%02d%02d%02d', $year-1900, $month, $day);

   ...
}