I'm working on a program that does tasks in tandem. It forks into some number of processes, and the children wait for specifically formatted text files to tell them exactly what to do. Since each text file should only be performed on once, I thought I should get an exclusive file lock on each file before actually doing the work. But files are sometimes worked on more than once despite the fact that an exclusive lock has returned true. Heres a simplified version of what I'm doing that recreates the error on my computer. 
#!/usr/bin/perl

use strict;
use warnings;
use Time::HiRes qw|time sleep|;
use Fcntl qw|:flock|;

mkdir "tmp" or warn $!;
chdir "tmp" or die $!;

for (1 .. 500){
    my $pid = fork();
    defined($pid) or die "fork failed: $!";
    if ($pid == 0) { 
        child_process();
        exit;
    }
}   
print "DONE FORKING\n";
while (wait() > -1) {}

sub child_process{
    while (1){
        while (my $file = glob "*.tmp"){
            open my $FH, '>', $file or next; #next if unlinked already
            if (flock($FH, LOCK_EX | LOCK_NB)){
                next unless flock($FH, LOCK_EX | LOCK_NB);
                print time(), " YES! I ($$) got a lock on $file!\n";
                sleep(.1); 
                unlink $file;
            }
            close $FH;
        }
        sleep(.1);
    }
}

[download]
Running this, waiting for the "DONE FORKING" message and than typing "touch tmp/random.tmp" produces something like this: 
1199321503.71728 YES! I (25930) got a lock on random.tmp!
1199321504.01631 YES! I (25827) got a lock on random.tmp!
1199321504.01702 YES! I (25976) got a lock on random.tmp!

[download]
I'm under the impression that there should only be one process that gets an exclusive lock.

In reply to flock LOCK_EX not locking exclusively by Anonymous Monk

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