gistfile1.txt

Mojo::IOLoop::Delay->new->steps(
  sub {
    my ( $delay ) = @_;
    $db->query( '...', cb => $delay->begin );
    $db->query( '...', cb => $delay->begin );
  },
  sub {
    my ( $delay, @results ) = @_;
    $c->render( ... );
  },
)->wait;

gistfile2.txt

my $delay = Mojo::IOLoop::Delay->new;
$db->query( '...', cb => $delay->begin );
$db->query( '...', cb => $delay->begin );
$delay->on( 'finish' => sub {
    my ( $delay, @args ) = @_;
    $c->render( @args );
} );
$delay->wait;

gistfile3.txt

my @collected = ();
my $waiting = 0;

$db->query( '...', cb => sub {
    my ( $db, @args ) = @_;
    push @collected, @args;
    $waiting--;
} );
$waiting++;

$db->query( '...', cb => sub {
    my ( $db, @args ) = @_;
    push @collected, @args;
    $waiting--;
} );
$waiting++;

Mojo::IOLoop->one_tick until $waiting == 0;

$c->render( @collected );