use strict;
sub create_obj {
  my $self = {};
  bless $self;
  return $self;
}

my $obj1 = create_obj();
my $obj2 = create_obj();
my $obj3 = create_obj();

$obj1->{DATE}="2005-01-01";
$obj2->{DATE}="2004-12-20";
$obj3->{DATE}="2005-01-02";

sub sort_rule {
  my ($year1,$month1,$day1) = split /-/,$a->{DATE};
  my ($year2,$month2,$day2) = split /-/,$b->{DATE};
  return $year1 <=> $year2 or
         $month1 <=> $month2 or
         $day1 <=> $day2
        ;
}

my @list = ($obj1,$obj2,$obj3);

print $_->{DATE},"\n" for sort sort_rule @list;