#!/usr/bin/perl -w
use strict;
use Fcntl ':flock'; # import LOCK_* constants

sub lock {
    flock(FH,LOCK_EX);
    # and, in case someone appended
    # while we were waiting...
    seek(FH, 0, 2);
}

sub unlock {
    flock(FH,LOCK_UN);
}

my $filename = "plain-db.pdb";

open(FH, ">>$filename")
    or die "Can't open file ($filename): $!";

lock();

# do what ever you want with it 

unlock();