use Fcntl;
{
    my {$in,$out);
    sysopen $out, "$path/$fname.bak",
              O_WRONLY | O_CREAT | O_EXCL | O_EXLOCK
        or sleep 1 and redo;
    open $in, '<', "$path/$fname" or die $!;
    flock $in, LOCK_EX;
    while (<$in>) {
        # edit operations to modify or skip $_
        print $out or die $!;
    }
    close $in;
    close $out or die $!
    rename "$path/$fname.bak", "$path/$fname" or die $!;
}