Part of the rational for modern relational database was to solve the problems caused by race conditions and data corruption when using flat file databases that are accessed by mutiple processes simultaneously. Using a database allows the database to handle these issues for you so you might do well to consider that option.

That said this code should (probably :-) work OK for you:

#!/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
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Update

Added seek FH, 0, 2 as per suggestion by dws

cheers

tachyon

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