#!/usr/bin/perl -w

use strict;
use POSIX ":sys_wait_h";
use Fcntl ':flock';
$|++;

# set a timeout in second waiting for an exclusive lock
my $timeout = 10;

# in main body of code open file for append
# filehandle will be available to all your kids
open FH, ">>$file" or die "Can't open $file: $!";

my $pid = fork()
die "Can't fork $!\n" unless defined $pid;
unless ($pid) {
    # in a kid, get an exclusive lock on file
    my $tries = 0;
    until (flock FH, LOCK_EX|LOCK_NB) {
        sleep 1;
        die "Can't get exclusive lock $!\n" if $tries++ > $timeout
    }
    seek FH, 0, 2;      # make sure we are at EOF
    print FH $blah;     # safe to print here   
    flock FH, LOCK_UN;  # release lock for other kids
    exit;               # kill kid
}

# wait for all kids to finish
my $kids;
do{ $kids = waitpid(-1, &WNOHANG) }until $kids == -1; 

close FH; # close file in parent