Well I would do it like the above and I guess the validation could be included also.

$date="24-02-2000";

if ($date =~ /(\d{2})-(\d{2})-(\d{4})/){
  $month=$1;$day=$2,$year=$3;
  print "ERROR month invalid",$month="01","\n" if($1 > 12);
  print "ERROR day invalid",$day="01","\n" if($2 > 31);
  print "ERROR year invalid",$year="2000","\n" if($3 > 2100 || $3 < 19
+00);
  $new_date = $year."-".$month."-".$day;
}
print "$new_date\n";
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You could be as fancy with the validation as you want.
tidy up the code as much as you desire