use Fcntl   qw( O_CREAT   O_EXCL   O_WRONLY   O_EXLOCK  );

$creat_flags = (O_CREAT | O_EXCL | O_WRONLY | O_EXLOCK );

sysopen(SOMEHANDLE, $somepath, $creat_flags, 0666)
    || die "$0: couldn't sysopen $somepath with flags $creat_flags: $!";