...
sub do_log {
    ...
    sysopen my $fh, $logfile, O_RDWR|O_CREAT or do {...};
    my $tmp_file = "${logfile}_tmp";
    open my $tmp_fh, '>', "$tmp_file" or do {...};
    if (flock $fh, LOCK_EX) {            #exclusive lock
        while (<$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
}