The listener: accept client connection 

sub accept_connection {
    my $self = shift;

    my $sock = $self->{_listener}->accept;
    $self->{_selector}->add($sock);

    my $key = refaddr($sock);
    $self->{$key} = { fh  => $sock, buf => '' };

    $sock->write(qq{200 READY\x0A});
}