use POSIX qw( strftime );

sub next_day {
   my $start_d = (localtime)[3];
   for (;;) {
      sleep(30*60);
      my $d = (localtime)[3];
      return if $d != $start_d;
   }
}

next_day() for 1..5;
print strftime("%Y-%m-%d\n", localtime());

next_day() for 1..4;
print strftime("%Y-%m-%d\n", localtime());