sub startup {
    my $self = shift;

    $self->helper(API => sub {
        ...
        if (no error) {
            return { result => { foo => 'bar' } };
        } elsif (error occured) {
            return { error => { foo => 'bar' } };
        }
    });

    my $r = $self->routes;
    $r->get('/')->to('app#main');
}

# at app.pm
sub main {
    my $self = shift;

    my $api = $self->API(args);
    if ($api->{'error'}) {
        $self->render('error/api', %{$self->{'error'}});
    }
    ...
    $self->render('app/main')
}