hirs vs pers

hirs

pron
  • That which belongs to hir, theirs (singular). Possessive case of hir, used in place of a noun. Coordinate with his and hers. 

pers

pron
  • That which belongs to per, theirs (singular): possessive case of per, used in place of a noun. 

How often have the words hirs and pers occurred in a corpus of books? (source: Google Ngram Viewer )