use strict;
use warnings;

sub sleep_time ($) {
   my ($start_std,$start_min,$start_sec) = split /:/, shift;
   $start_sec += $start_min * 60 + $start_std * 3600;
   my ($sec,$min,$std) = (localtime)[0..2];
   $sec += $min * 60 + $std * 3600;
   return $start_sec > $sec ? $start_sec - $sec : 86399 - $sec + $start_sec;
}

my $start_time = '16:00:00';

while (1) {
   sleep sleep_time($start_time);
   werteeinlesen();
   pruefen();
}