If you use this code within your outer loop (the one with $cv->begin...$cv->end, then I think you're calling $cv->end too early and you never ->recv the guards, so I'm not sure that you ever enter the AnyEvent event loop...

I think the intended usage of $cv->begin ...$cv->end is that the ->end call happens within the callback, not within the loop body:

my $all_requests_done = AnyEvent->condvar;
for my $r (@requests) {
    $all_requests_done->begin();
    http_post $r, sub {
        ...
        $all_requests_done->end();
    };    
};
$all_requests_done->recv;
[download]

Can you try to do a simple, self-contained program that does a single HTTP POST request and waits for it, without the ->begin ... ->end stuff?

Update: (After two reboots due to power loss) added intended usage of ->begin ... ->end