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;
}