Validation rules that depends on what the request is using condition

You could use the request helper:

public function rules()
{
    $baseRules = [
       'email' => 'required|email',
       'g-recaptcha-response' => 'required|recaptcha'
    ];
    if(request()->get("valueFromPost") === '2') {    
        return $baseRules + [                
            'confirm_email' => 'required|email|same:email'
        ];

    }

    return $baseRules;
}

Or the shorter (but less readable) version:

public function rules()
{
    return [
       'email' => 'required|email',
       'g-recaptcha-response' => 'required|recaptcha'
    ] + (request()->get("valueFromPost") === '2' ? [ 'confirm_email' => 'required|email|same:email' ] : []);    
}