#!/usr/bin/perl
use warnings;
use strict;

use Fcntl qw(:flock :DEFAULT);

my $pid = fork;
die "Cannot fork.\n" unless defined $pid;

if ($pid) {
    open my $parent, '>', 'tmp'       or die "open parent: $!";
    sleep 1;
    flock   $parent, LOCK_EX          or die "lock parent: $!";
    print  {$parent} "Parent\n";
    close   $parent                   or die "close parent: $!";

} else {
    open my $child, '>', 'tmp'        or die "open child: $!";
    flock   $child, LOCK_EX           or die "lock child: $!";

    # Longer than parent:
    print  {$child} "Child here...\n";
    close   $child                    or die "close child: $!";
}