#!/usr/bin/perl

use Fcntl qw(:DEFAULT :flock);
use strict;

my $FH;
my $fn = 'flockdata';

my $test_data = (join('','a'..'z')."\n") x 10;
#print length($test_data),"\n";

sysopen($FH,$fn,O_WRONLY|O_CREAT);

for(1..10) {
  last unless fork();
}

for(1..20) {
  die "FLOCK ERROR\n" unless flock($FH,LOCK_EX);
  die "SYSWRITE ERROR\n" unless syswrite($FH,$test_data,length($test_d
+ata)) == \
length($test_data);
  flock($FH,LOCK_UN);
}

close $FH;
[download]

perl -e 'system(q{perl testflock.pl ; ls -al flockdata; rm flockdata})
+ for 1..20'
[download]

As I mentioned before you must put a sysseek in there (e.g. after the flock, but before the syswrite). If you don't you will see corruption since a single process will have a stale version of the current EOF for it's file descriptor.

I also see corruption with your code on my mac. I don't see it if there is a sysseek($FH, 0, 2). You mentioned that sysseek() "broke everything". When you put it back in this code, and check for error returns, what do you see?

If there were no failure conditions for flock, then I'm sure the documentation wouldn't say

Returns true for success, false on failure

I'm not quite sure what the exact failure condition is in which flock neither waits for the lock nor dies, but it seems to exist.

(Update: It seems that it only will fail if LOCK_NB is used, but I'm not entirely sure).

Yeah, flock can fail, but that's typically a case of running out of system resources; possibly, but rare. It doesn't seem the OP is describing a case that happens only third blue moon.

It doesn't mean you shouldn't check the return value of flock, but I doubt that this is the cause of the problem.