my $fetch;
$fetch = sub {

  # Stop if there are no more URLs
  return unless my $url = shift @urls;

  # Fetch the next title
  my $end = $delay->begin;
  $ua->get($url => sub {
    my ($ua, $tx) = @_;
    say "$url: ", $tx->result->dom->at('title')->text;
    $end->();

    # Next request
    $fetch->();
  });
};

# Process two requests at a time
$fetch->() for 1 .. 2;
$delay->wait;

##</code><code>##

$fetch->();
$delay->wait;