Trying to wrap my head around event programming, I ran into this error. My script:

#perl

use AnyEvent;

my $input_waiting = AnyEvent->condvar;
my $output_waiting = AnyEvent->condvar;

my $input_from_stdin = AnyEvent->io (
    fh   => \*STDIN, # which file handle to check
    poll => "r",     # which event to wait for ("r"ead data)
    cb   => sub {    # what callback to execute
        my $input = <STDIN>; # read it
        push(@input, $input);
        $input_waiting->send;

        $output_waiting->recv;
        print shift(@output) ."\n";
    }
);


while(1){
    # this "blocks" (while handling events) till the callback
    # calls ->send
    $input_waiting->recv;

    ## do some possibly lengthy processing
    push(@output, 'done');
    $output_waiting->send;

    ## finish this cycle,
    ## AnyEvent docs say: "You can only wait once on a condition - add
+itional calls are valid but will return immediately."
    ## but we need to stop this loop again, so this:
    $input_waiting = AnyEvent->condvar;
}
[download]

As you can see, I am trying to implement a simple Input->process->Output mechanism, like for example chatbots use it. Where the printing of the output should be delayed as long as the processing takes. Is my overall design design okay for that?

Second, as I understand the condvar/signalling logic in AnyEvent, I can use a ->rcv once to halt my processing loop, but after that I can't tell it to go back to sleep as "You can only wait once on a condition". So I need to redeclare a new condvar - is that right?

Third, I need to halt my processing until some input arrives in @input. By doing a ->send in my input waiter I trigger the processing to take place. But how do I halt the input waiter until @output contains some value?
I can't return the @output from some other point in my code as what it is done here via STDIN/STDOUT may as well be a http connected client where I can't simply append data. I need to halt the code in $input_from_stdin for as long as the processing needs so I keep this "sessing" alive there.

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