use AE             qw( );
use AnyEvent::HTTP qw( http_get );

sub make_request {
   my ($url, $cb) = @_;
   
   http_get($url,
      ...,
      sub {
         ...
         $cb->();
      },
   );
}

my $max_requests = 10;
my $active_requests = 0;

my $cv = AE::cv();
while (my $url = get_next_url()) {
   while ($active_requests == $max_requests) {
      $cv->recv();
      $cv = AE::cv();
   }

   ++$active_requests;
   make_request($url, sub {
      --$active_requests;
      $cv->send();
   });
}

while ($active_requests) {
   $cv->recv();
   $cv = AE::cv();
}