NoDictionaries   Text notes for  
... seu propria formidine, dum per simplice[m] victu[m] et agrestibus...

"[P]er" is not the preposition; it's a prefix forming the compound adjective in the ablative, "persimplici," appearing only here; the phrase is "persimplici victu," "with very simple food," describing how Seneca satisfied hunger, perhaps out of fear of being poisoned.