Dative in the Hit. peruni. Also the Sansk. párvata- 'rock, mountain'. In the meaning of 'oak': Longob. fereha, New High Germ. vereiche, Swed. ferch 'oak wood', Celt. ἕρκος 'oak wood' (in the Greek rendering). Иванов, Гамкрелидзе 1984, 613-614

For *perkʷ- 'oak' see

Matasović, Ranko (2009), “*kʷerxt-”, in Etymological Dictionary of Proto-Celtic (Leiden Indo-European Etymological Dictionary Series; 9), Leiden: Brill, page 178
Mallory, J. P. with Adams, D. Q. (2006), The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World (Oxford Linguistics), New York: Oxford University Press, page 160

De Vaan, Michiel (2008), “quercus”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, page 506