Route::post('/',function(){

  //We first define the Form validation rule(s)
  $rules = array(
    'link' => 'required|url'
  );

  //Then we run the form validation
  $validation = Validator::make(Input::all(),$rules);

  //If validation fails, we return to the main page with error info
  if($validation->fails()) {
    return Redirect::to('/')
        ->withInput()
        ->withErrors($validation);
  } else {
    //Now let's check that if we have the link already in our database, if so we get the first result
    $link = Link::where('url','=',Input::get('link'))
      ->first();
    //If we have the URL saved in our database already, we provide that information back to view.
    if($link) {
      return Redirect::to('/')
        ->withInput()
        ->with('link',$link->hash);
    //Else we create a new unique URL
    } else {
      //First we create a new unique Hash
      do {
        $newHash = Str::random(6);
      } while(Link::where('hash','=',$newHash)->count() > 0);

      //Now we create a new database record
      Link::create(array(
        'url' => Input::get('link'),
        'hash'  => $newHash
      ));

      //And then we return the new shortened URL info to our action
      return Redirect::to('/')
        ->withInput()
        ->with('link',$newHash);
    }
  }
});

route code

I suggest using named routes and controllers this code is really unreadable.

Route::get('/', array('as' => 'home', 'uses' => 'Controller@getHome'));