How to instantiate an asset bundle prefab after it's been loaded?

I want to load a prefab from an asset bundle and instantiate the prefab some time later in the game rather than as soon as the prefab is loaded. Here’s my code for loading the prefab:

IEnumerator LoadAssetBundle(string url)
{
    WWW www = WWW.LoadFromCacheOrDownload(url, 1);
    print("Loading www");

    // Wait for download to complete
    yield return www;
    if (!string.IsNullOrEmpty(www.error))
    {
        Debug.LogError(www.error);
        yield return null;
    }

    AssetBundle bundle = www.assetBundle;
    AssetBundleRequest request = bundle.LoadAssetAsync("Goblin", typeof(GameObject));

    // Wait for completion
    yield return request;

    // I don't want to instantiate like this right now
    //GameObject obj = request.asset as GameObject);
    //Instantiate(obj);

    bundle.Unload(false);
    www.Dispose();
}

Here’s where I want to later instantiate the prefab. I just can’t figure out this part.

void SomeFunction()
{
    // instantiate the goblin loaded from the asset bundle here
}

I don’t want to have to call that huge www function again, I just want the simple GameObject.Instantiate() function. Any ideas?

Based on dorpeleg’s answers, I created a prefab called PrefabManager, tagged it (for speed) and added the following script:

public class PrefabManager : MonoBehaviour
{
    Dictionary<string, GameObject> prefabsDictionary_;

    void Awake()
    {
        prefabsDictionary_ = new Dictionary<string, GameObject>();
    }

    public void AddPrefab(GameObject prefab)
    {
        prefabsDictionary_.Add(prefab.name, prefab);
    }

    public static GameObject Spawn<T>() where T : MonoBehaviour
    {
        string prefabName = typeof(T).Name;
        GameObject prefabManagerGameObject = GameObject.FindGameObjectWithTag(typeof(PrefabManager).Name);
        PrefabManager prefabManager = prefabManagerGameObject.GetComponent<PrefabManager>();
        if (prefabManager.prefabsDictionary_.ContainsKey(prefabName))
        {
            GameObject original = prefabManager.prefabsDictionary_[prefabName];
            GameObject clone = GameObject.Instantiate(original);
            return clone;
        }
        else
        {
            Debug.LogError("No matching runtime prefab found with name: " + prefabName);
            return new GameObject();
        }
    }
}

And instantiate prefabs like this:

void SomeFunction()
{
    PrefabManager.Spawn<Goblin>();
}

I’m marking dorpeleg’s as answer because it’s the concept in a more simple form of my answer.

All you need to do is keep a reference to it:

private GameObject _myPrefab;

// inside IEnumerator LoadAssetBundle(string url) ...
_myPrefab = request.asset as GameObject;

 void SomeFunction()
 {
     Instantiate(_myPrefab );
 }