get '/' => needs login => sub {
    # ...
};

get '/login' => sub {
    template 'login'
};

post '/login' => sub {
    my $user = body_parameters->get('uname');
    if (is_valid($user, body_parameters->get('psw'))) {
        session(user => $user);
        redirect('/')
    } else {
        template index => {error => 'Wrong username or password'}
    }
};