Not all days have 24 hours. Fix below. This will also give you the answer up to 23.5 hours sooner.

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());
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Suffers from a race-condition that manifests itself if the loops is started very shortly before midnight.