Here's some sample code that demonstrates the problem:

#!/usr/bin/perl -w

use Fcntl ':flock';
use POSIX "sys_wait_h";

my $file = '/tmp/test-file.txt';
my $try;

my $pid = fork();
if ($pid == 0) {
        #this is the child
        #Go to sleep and let the parent lock the file
        print "Child ($$) going to sleep for a bit...\n";
        sleep(3);
        print "Child ($$) woke up!\n";
        #Wake up and try to open and lock the file
        print "Child ($$) attempting to open for writing...\n";
        if (open CWRITER, ">$file"){
                print "Child ($$) completed open for writing\n";
                print "Child ($$) closing filehandle.\n";
                close CWRITER;
                print "Child ($$) closed filehandle.  Exiting.\n";
        } else {
                print "Child ($$) unable to open for appending: $!\n";
        }
        exit;
} else {
        #this is the parent
        #open a filehandle and lock it with LOCK_EX
        print "Parent ($$) attempting to open file\n";
        if (open PWRITER, ">$file"){
                print "Parent ($$) opened file, attempting LOCK_EX\n";
                my $try = flock(PWRITER, LOCK_EX) or warn "Parent ($$)
+ can't flock: $!\n";
                print "Parent ($$) try for LOCK_EX is '$try'\n";
                print "Parent ($$) printing to file\n";
                print PWRITER "Parent says hello.\n";
                print "Parent ($$) done writing, closing filehandle.\n
+";
                close PWRITER;
                print "Parent ($$) closed filehandle, now reopening as
+ reader.\n";
                if (open PREADER, "$file") {
                        print "Parent ($$) opened file for reading.  R
+equesting LOCK_EX.\n";
                        flock(PREADER, LOCK_EX) or warn "Parent ($$) u
+nable to LOCK_EX: $!, $?\n";
                        print "Parent ($$) got LOCK_EX.\n";

                        print "Parent ($$) going to sleep.\n";
                        sleep(10);
                        print "Parent ($$) woke up!\n";

                        print "Parent ($$) reading now...\n";
                        my @lines = <PREADER>;
                        print "Parent ($$) read:\n@lines\n";
                        print "Parent ($$) closing PREADER\n";
                        close PREADER;
                        print "Parent ($$) closed PREADER.\n";
                } else {
                        print "Parent unable to open file for read: $!
+\n";
                }

        } else {
                warn "Parent ($$) couldn't open '$file' for reading: $
+!\n";
        }

        my $waiter = 1;
        while ($waiter > 0){
                #try to reap the child PID
                $waiter = waitpid($pid, &WNOHANG);
                if ($waiter == -1){
                        print "Waiter got -1: $?\n";
                } elsif ($waiter == 0) {
                        print "Waiter got 0, $pid still alive...\n";
                } elsif ($waiter > 0) {
                        print "Waiter got $waiter, $?\n";
                } else {
                        print "Waiter got something odd: '$waiter','$?
+'\n";
                }
                sleep(2);
        }
        print "Parent ($$) done reaping child $pid\n";
}

print "$$ Exiting\n";
[download]

And here's what happens when I run it:

bash-2.05a$ ./test.pl
Parent (12352) attempting to open file
Parent (12352) opened file, attempting LOCK_EX
Parent (12352) try for LOCK_EX is '1'
Parent (12352) printing to file
Parent (12352) done writing, closing filehandle.
Parent (12352) closed filehandle, now reopening as reader.
Parent (12352) opened file for reading.  Requesting LOCK_EX.
Parent (12352) got LOCK_EX.
Parent (12352) going to sleep.
Child (12353) going to sleep for a bit...
Child (12353) woke up!
Child (12353) attempting to open for writing...
Child (12353) completed open for writing
Child (12353) closing filehandle.
Child (12353) closed filehandle.  Exiting.
Parent (12352) woke up!
Parent (12352) reading now...
Parent (12352) read:

Parent (12352) closing PREADER
Parent (12352) closed PREADER.
Waiter got 12353, 0
Waiter got -1: -1
Parent (12352) done reaping child 12353
12352 Exiting
bash-2.05a$