use Fcntl qw(O_WRONLY O_CREAT O_EXCL);
my $lockfile = '/tmp/lock';
if (sysopen(FH, $lockfile, O_WRONLY | O_CREAT | O_EXCL)) {
    # sysopen will only work if lockfile does not exist
    # do stuff
    close FH;
    unlink $lockfile or die "Can't unlink lockfile $lockfile $!";
}
else {
    die "$lockfile exists so another instance of the script is running!";
  exit;
}