JavaFan,
The code shown is the extract from the slides presented by Dominus. I provided it here so people wouldn't have to go to an offsite location to see them. Here is complete running code that demonstrates the problem:

#!/usr/bin/perl
use strict;
use warnings;
use Fcntl qw/:DEFAULT :flock/;

open(SELF, '<', $0) or die "Unable to open '$0' for reading: $!";
flock SELF, LOCK_EX | LOCK_NB or exit;

print "I must be the only process running\n";
sleep 300;
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and

#!/usr/bin/perl
use strict;
use warnings;
use Fcntl qw/:DEFAULT :flock/;

flock DATA, LOCK_EX | LOCK_NB or exit;

print "I must be the only process running\n";
sleep 300;
__DATA__
[download]