use Fcntl qw(:flock);

open(LOCK, ">>", "lock")
        or die "Error: could not open or create lock: $!";

print "Waiting for lock...\n";
flock(LOCK, LOCK_EX)
        or die "Error: could not get lock";
print "Got lock!\n";

print "Working exclusively...\n";
sleep(10);
print "Done.\n";

print "Release lock.\n";
flock(LOCK, LOCK_UN);
print "Done.\n";

close(LOCK);