use Fcntl qw( LOCK_EX O_RDWR O_CREAT SEEK_SET );

my $qfn = '/tmp/myfile.index';

sysopen(my $fh, $qfn, O_RDWR|O_CREAT, 0666)
   or die("Can't create or open \"$qfn\": $!\n");
flock($fh, LOCK_EX);

my $index = <$fh>;
++$index;

seek($fh, 0, SEEK_SET);
print($fh "$index\n");
#truncate($fh, tell($fh));