use strict;
use Fcntl qw(:DEFAULT);
my $semaphore = 'c:/test.txt';

sysopen (SEMAPHORE, $semaphore, O_WRONLY | O_EXCL | O_CREAT) or &busy;

print "There can be only one!";
sleep 20;
close SEMAPHORE;
unlink $semaphore or die "Can't unlink $semaphore $!\n";
exit;

sub busy {
    print "Sorry, I am busy!";
  exit;
}

##</code><code>##

C:\>type flock.pl

use Fcntl qw(:flock);

print "This is $^O\n";

open  FILE, ">>test.txt" or die "Oops $!\n";
flock FILE, LOCK_EX or die "Can't flock $!\n";
close FILE;

C:\>perl flock.pl
This is MSWin32
flock() unimplemented on this platform at flock.pl line 6.

C:\>