From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger observes that many people improperly touch an uncovered mezuzah parchment with bare hands, without distinguishing between a mezuzah in a case and one whose parchment is exposed. He rules that touching an uncovered mezuzah bare-handed is prohibited based on the dictum of Rabbi Parnach, which Tosfos and the Rambam extend to all kisvei hakodesh, and that doing so constitutes a mitzvah haba'ah ba'aveirah since the custom of touching the mezuzah derives only from a post-Talmudic source. He recommends pulling one's sleeve over one's hand to touch the mezuzah through the garment.
Source: Shu"t Rabbi Akiva Eiger §58