use Fcntl qw(:flock :seek);
open my $fh, "+<", "datei" or die $!;
flock $fh, LOCK_EX;
seek $fh, 0, SEEK_SET;
chomp(my $count = <$fh>);
$count++;
seek $fh, 0, SEEK_SET;
truncate $fh, 0;
print $fh "$count\n";
close $fh;