#!/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;

##</code><code>##

#!/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__