Look more closely at how you're obtaining a lock:

1. You are opening a lock to a semaphore file (lock.tmp).
2. Once that lock is obtained you are opening an output file (somefile$_.tmp).
3. You unlock/close your semaphore file.
4. You write to somefile$_.tmp
5. You close somefile$_.tmp.
6. You copy from your tmp file to a new file.

Steps 4, 5, and 6 are unprotected.

You should not be releasing your semaphore file lock until after you're done writing to somefile$_.tmp, and in fact, probably not even until after the copy operation. And probably shouldn't be using LOCK_UN as a matter of habit (simply closing a filehandle will terminate the lock, and explicitly unlocking before closing can create a race condition, though not in this usage.)

You may already know about this set of slides from Mark Jason Dominus, but just in case: File Locking Tricks and Traps.

1. You are opening a lock to a semaphore file (lock.tmp).
2. Once that lock is obtained you are opening an output file (somefile$_.tmp).
3. You unlock/close your semaphore file.
4. You write to somefile$_.tmp
5. You close somefile$_.tmp.
6. You copy from your tmp file to a new file.
Steps 4, 5, and 6 are unprotected.

I knew that if lock extended to other steps it works, but I could not understand why.

The thing is: When I open file on step (2), I write to it in step (4). But steps (4) and (5) are actually protected. Steps (4) and (5) is performed only if that process was the one who created file. Otherwise step is skipped. (and this was intentional - I was trying to minimize lock time which was important)

Notice "if ($f)"


 if ($f) {
  print ($f "x") for (1..40_000_000);
  close $f;
 }
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Problem was that step (6) unprotected. And this assertion was wrong:

die if -s $filename != -s $newfilename;

Correct assertion would be:

die if 40_000_000 != -s $newfilename;

i.e. when data copied, file could be in the middle of creation by another process. so in the end I see:
correct size of $filename
wrong size of $newfilename
assertion passed: if -s $filename != -s $newfilename;

Thank you! I'll mark post as SOLVED

Notice "if ($f)"

I suspect this test does not do what you think it does. In particular, consider the case that one process opens the file and writes to it, then another process opens the same file and, perhaps, writes to it, then the if ($f) test executes in the first process. Is the result any different because of what the other process did? I think you will find it is not and, therefore, that the statements in the if block are not as protected as you think they are.

Whether your problem is solved depends on what you are trying to do, which you don't say, but what you are calling a solution seems strange to me. I suggest you reconsider.

Running your program on Windows several times, each trial I get a variable subset of the 25 potential output files with names beginning with 'c' and several of them have less than 40_000_000 characters in them.

1. file size of some c_* files is less than 40000000, however this should not happen, because of line: die if -s $filename != -s $newfilename;

The die will not prevent the production of files with less than 40_000_000 characters because the test is performed after the copy, not before. Even if you moved the test before the copy, because these statements are not protected by the lock file, the test could pass but another process could modify the source file before the copy executes, resulting in a file of different length (and, perhaps, content if in your real program the different processes write different content).

Changing the test on the die to 40_000_000 != -s $newfilename makes little difference to the outcome: there is still a variable subset of the possible 25 copied files and there are still several of them with fewer than 40_000_000 characters. Of course, it makes a difference as, when the test runs, the size of $filename might be other than 40_000_000. This will be somewhat random, depending on how execution of the various processes is interleaved. But, perhaps this is exactly as you intend and I am worrying for nothing.

I'm not entirely sure what you are trying to achieve, but the issue seems to be that you are only locking the file open not the file writes.

 so :-
fork 1                      fork 2

takes the lock
opens file1
releases the lock
starts writing data
gets pre-empted
                            takes the lock
                            opens file1 (& deletes contents)
                            releases the lock
                            starts writing data 

Etc....
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So potentially many children all writing to the same file at the same time. If you get each fork to write a different character to the file you'll see what's happening.

It's usually better to get each child to write to its own temp file, File::Temp, then rename it.

So potentially many children all writing to the same file at the same time.

See above Re^2: Strange IO + concurrency issue - write-to file is actually protected (although it's extremely unclear in code). What was not protected is file copy (where destination file is unique)

It's usually better to get each child to write to its own temp file

Same, in this case there are no concurrent writes to same file (even copy writes to unique file).