I don't think that Prof. Karp was asking if there was a scriptural sanction.
He was just enquiring about the availability of the motif.
Such motifs when available in folk tales and fairy tales are not considered to be reflecting the cultural reality of the tradition bearers. In Jung's classification (1. Psychological and 2. Visionary) , these narratives form part of visionary which means they contain dreamlike elements which have complex mechanisms of 'symbolism', related to unconscious aspects such as guilts, fears etc.
That said, there are certain fairy tales particularly those involving sorcery, witchcraft and the like, where such motifs involving horror and morbidity, containing formulae of sacrificing or consuming organisms for acquiring the powers of the sacrificed or consumed organism do occur in India and many other parts of the world. Bhetala pancha vimsati etc, have such motifs. A trickster teacher conspiring to sacrifice the disciple being sacrificed by the disciple who realizes the conspiracy of the trickster teacher is found in Vikramarkacharitra.