sub walk {
     my ( $job, $result, $failed ) = @_;

     # Yielding is critical when running an event loop in parallel.
     # Not doing so means that the app may reach contention points
     # with the firewall and likely impose unnecessary hardship at
     # the OS level. The idea here is not to have multiple workers
     # initiate HTTP requests to a batch of URLs at the same time.
     # Yielding in 1.827+ behaves more like scatter for the worker
     # to run solo in a fraction of time.

     MCE::Hobo->yield( 0.03 );   # MCE::Hobo 1.827

     my $cv = AnyEvent->condvar();

     # Populate the hash ref for URLs it could reach.
     # Do not mix AnyEvent timeout and Hobo timeout.
     # Choose to do the event timeout if available.

     foreach my $url ( @{ $job->{INPUT} } ) {
         $cv->begin();

         http_get $url, timeout => 2, sub {
             my ( $data, $headers ) = @_;
             $result->{$url} = $data;
             $cv->end();
         };
     }

     $cv->recv();

     # Populate the array ref for URLs it could not reach.

     foreach my $url ( @{ $job->{INPUT} } ) {
         push @{ $failed }, $url unless (exists $result->{ $url });
     }

     return;
 }