Ok, now I see the reason.

...
sub do_log {
    ...
    sysopen my $fh, $logfile, O_RDWR|O_CREAT or do {...};
    my $tmp_file = "${logfile}_tmp";
    if (flock $fh, LOCK_EX) {            #exclusive lock
        open my $tmp_fh, '>', "$tmp_file" or do {...};
        while (<$fh>) {
            ..
            my $line = $_;
            ..
            print $tmp_fh $line, "\n";
        }
        close $tmp_fh or die "Can't close tmp_file: $!";
        move($tmp_file, $logfile);
    }
    close $fh or die "Can't close logfile $!";  #unlocks automatic
}
[download]

A second question: I call a sub check_nr_of_lines($nr_ofl, $arch_path, $logfile, $fh); When the max nr of lines in the logfile is reached, I want to write the half nr of lines in an archive file and delete them in the logfile. The other half should still be in the logfile. The following code does not work. It creates only an empty archive file, and changes nothing in the logfile. What must I change?

...
sub do_log {
    ...
    sysopen my $fh, $logfile, O_RDWR|O_CREAT or do {...};
    my $tmp_file = "${logfile}_tmp";
    if (flock $fh, LOCK_EX) {            #exclusive lock
        open my $tmp_fh, '>', "$tmp_file" or do {...};
        while (<$fh>) {
            ..
            $nr_ofl = $.;
            my $line = $_;
            ..
            print $tmp_fh $line, "\n";
        }
        ...
        check_nr_of_lines($nr_ofl, $arch_path, $logfile, $fh);
        ...
        close $tmp_fh or die "Can't close tmp_file: $!";
        move($tmp_file, $logfile);
    }
    close $fh or die "Can't close logfile $!";  #unlocks automatic
}

sub check_nr_of_lines {
    my ($nr_of_lines, $arch_path, $logfile, $fh) = @_;
    if ( $nr_of_lines >= max_nr_of_lines_per_logfile ) {
        eval { mkpath($arch_path) };
        if ($@) {
            ...
        }
        my $c = int($nr_of_lines / 2);
        open my $fh3, '>', "$arch_path/arch_test.log" or do {
             ...
        };
        seek($fh, 0, 0);
        while (<$fh>) {
            print;
            if ($. <= $c) {
                print $fh3 $_;
                last;
            }
            print $fh $_;
        }
    }
    return;
}
[download]