Fred Smith wrote...
Yes, we're saying it won't work for a perpetuity. There has to be a finite
number of cash flows for IRR to calculate the return.

If your cash flow is level forever, the IRR is simply:

=CashFlow/InitialInvestment
....

Let's try that again.

Excel's IRR function breaks down for very long cashflows. However,
there's always Solver to find the interest rate at which the NPV
function returns zero. That said, arbitrarily long cashflows become
pure guesswork after some finite point - 5 years, 10 years, 50 years.
After that, if the cashflows were based on a constant arithmetic or
geometric progression, it's possible to calculate the PV at the time
that progression begins based in interest rate i. Add that term to the
NPV term, also using i, then use Solver to set the whole thing to zero
by varying i.