async sub fetch {
   my ( $self, $params ) = @_;

   my $loop = IO::Async::Loop->new;

   my $received = $loop->new_future();                    # <---
   my @received;

   my $client = Net::Async::WebSocket::Client->new(
      on_text_frame => sub {
         my ( $clientself, $frame ) = @_;

         push @data_received, $frame;

         if ( $num_items_received >= $MAX_HELLOS ) {
            $received->done();
         }
      }
   );

   $loop->add( $client );

   defer { $client->closenow; }

   ...;  # $client->connect.

   ...;  # $client->send_text_frame loop.

   await $received;                                       # <---

   return \@data_received;
}