use Fcntl qw(:DEFAULT :flock);

sysopen(OUT, $outfile, O_WRONLY | O_CREAT)
   or die "Cannot open $outfile for writing:$!\n";
flock(OUT, LOCK_EX)
   or die "Cannot get a lock on $outfile:$!\n";
truncate(OUT, 0)
   or die "Cannot truncate $outfile:$!\n";