Well, here is an example I ran on the redhat boxes at work and my debian box at home, it shows bad data on both systems if run enough (usually 20 iterations will do it). I also added a check for syswrite... doesn't seem to be writing short data.

#!/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;
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and to run it when named testflock.pl

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