Mixing Coro and AnyEvent makes very little sense in your case.

I have no idea why you say that.

Making a fixed number of HTTP requests in parallel:

use AE             qw( );
use AnyEvent::HTTP qw( http_get );
use Coro           qw( async );
use Coro::Channel  qw( );

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

my $num_workers = 10;

my $q = Coro::Channel->new();

my @threads;
for (1..$num_workers) {
   push @threads, async {
      while (my $url = $q->get()) {
         my $cv = AE::cv();
         make_request($url, $cv);
         $cv->recv();
      }
   }
}

while (my $url = get_next_url()) {
   $q->put($url);
}

$q->shutdown();
$_->join() for @threads;
[download]

Sure, you can do it without Coro too.

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();
}
[download]

Whatever floats your boat.