#!perl -w
use strict;

use Fcntl qw(LOCK_EX LOCK_NB);
my $lockfile = "/someabsolutepath/writeabledirectory/lockfile";
open LOCKFILE, ">", $lockfile or die  "Cannot open lockfile\n";

if (!flock(LOCKFILE, LOCK_EX|LOCK_NB)) {
  print STDERR "Another copy of application is already running\n";
  exit(0);
}