#!/usr/local/bin/perl -w

use strict;

use Fcntl qw/:flock/;

my $file = 'flock.test';

-e $file or open(FILE, ">$file") or die "Can't create $file: $!";

if (fork) {
    open(FILE, "<$file") or die "Can't open $file for reading in parent: $!";
    flock(FILE, LOCK_EX) or die "Can't flock $file in parent: $!";
    wait;
} else {
    sleep 1;
    open(FILE, "<$file") or die "Can't open $file for reading in child: $!";
    flock(FILE, LOCK_EX|LOCK_NB) or die "Can't flock $file in child: $!";
}