$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;

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

sub unlock {
    flock MBOX, $LOCK_UN;
}

open MBOX, ">>/usr/spool/mail/$ENV{'USER'}"
    or die "Can't open mailbox: $!";

lock();
print MBOX $msg, "\n\n";
unlock();