my @sorted_dates = map  { $_->{'date'} }
                   sort { $a->{'year'}  <=> $b->{'year}  or
                          $a->{'month'} <=> $b->{'month} or
                          $a->{'day'}   <=> $b->{'day'}     }
                   map  { /(\d\d)-(\d\d)-(\d\d\d\d)/ ;
                          { 'date'  => $_,
                            'day'   => $1,
                            'month' => $2,
                            'year'  => $3 } } @start_date;

my $minimum_date = $sorted_dates[-1];