sub afork (@$&) {
    # First field = array, second field = max number of processes to
    # run at the same time, third field = subroutine to run against
    # each array element
    my ($data, $max, $code) = @_;
    my $c = 0;
    foreach my $data (@$data) {
        wait unless ++ $c <= $max;
        die "Fork failed: $!\n" unless defined (my $pid = fork);
        exit $code -> ($data) unless $pid;
    }
    1 until -1 == wait;
}

while(1) {
    @FILES = `ls -1 $DIRECTORY`;
    afork (\@FILES,3,\&process_file);
}