package HighLander;
use strict;
use Fcntl qw( LOCK_EX LOCK_NB );

open(our $fh, '<', $0)
  or die("Can't open \"$0\": $!\n");

unless ( flock($fh, LOCK_EX|LOCK_NB )) {
  print "Another instance of \"$0\" is already running. Exiting...\n";
  exit(0);
}

1;

##</code><code>##

#!/usr/bin/perl
use strict;
use HighLander;

# if this script is already running, you'll never make it this far... 
print "Running...\n";
sleep 10;

##</code><code>##

if ($! == ($^O =~ /Win32/ ? 33 : EWOULDBLOCK)) {

##</code><code>##

use Highlander qw( :verbose );

##</code><code>##

...

unless ( flock($fh, LOCK_EX|LOCK_NB )) {

  if ($verbose) {
    print "Another instance of \"$0\" is already running. Exiting...\n";
  }
  exit(0);
}

...