You are assuming that the meaning of the words "perpetual" and "irrevocable" are given outside the context of their use in the contract. Which is what I am saying is a methodological error.Saying a contract is perpetual, though, doesn't automatically mean irrevocable. You need to look at more to figure it out. A contract that is saying that it is irrevocable is pretty clear about what it means.
I mean, here's one natural-language meaning of "perpetual": will last forever. And here's one natural-language meaning of "irrevocable", in the context of an agreement or arrangement: unable to be brought to an end by an act of will.
On those meanings, then, because will last forever entails is not able to be brought to an end, and a special case of is not able to be brought to an end is is not able to be brought to an end by an act of will, something's being perpetual would entail it being irrevocable.
Of course, the natural language meanings I've canvassed aren't the only way those words can be used. But arguments about the variety of natural language meanings, and the logical relations between them, won't settle the interpretation of any particular contract. That's a matter of construction of the terms of that contract in their context.