Depending on whether the errant pencil is floating or not, you might find it useful to raise one wing or the other to get it towards the filler neck. Preferably lift the wing at the jacking point or if that isn't possible, on the mainspar line of rivets.
If you need to change the longitudinal access of the aircraft, PA38s are quite easy to lower the tail/raise the nose. Gently pull down on the leading edge of the fin - but not the plastic fairing.
And anything you use to look inside the tank with - tie string round it so if it does get dropped you don't end up with more than the pencil inside....