You start the get request asynchronouslyand then fall off the end of the script before it has completed,hence the error.

Try waiting for the response:

use HTTP::Async;
use HTTP::Request;
my $async = HTTP::Async->new;
$async->add( HTTP::Request->new( GET => 'http://www.perl.org/' ) );
my $response = $async->wait_for_next_response;
[download]

Of course, unless you add several gets, that is the same as doing a synchronous get, but it will 'fix' the error.