A sentence in Corderii Colloquia 24,
ille spiritus bonus faxit.
is translated as:
May that good spirit grant it.
How does the pf ind come to have an optative sense here?
It's actually not indicative, but subjunctive. I know Perseus' morph tool parses it as both indicative and subjunctive, but both Gildersleeve and the OLD say it's subjunctive and do not mention anything about it being indicative:
faxo, faxim (where later writers use fecero, fecerim)
The normal perfect indicative of facio was feci, fecisti, fecit (etc.).
faxim is (according to one theory) the subjunctive (historically: optative) of the old s-aorist; note that Old Latin also had an s-future faxō. There is a rather convoluted discussion of this in Sihler §502.