Is it
“Perhaps the grass were red”
Or
“Perhaps the grass was red”?
In your example, were. “If the grass were red, the war might have been won.” Subjunctive. “If the grass was red, it may have been bled upon.” A contingency in the past tense.
“Ah, wilderness were paradise enow.”
Why would “perhaps” take a subjunctive? I have never heard of that.
The subjunctive is used when the clause is known to be false. “Perhaps” or “maybe” leaves that in doubt.
If the grass were red, I would not fertilize it.
Perhaps the grass was red, but I didn’t get that good a look at it.
To keep tenses consistent, it would be “If the grass had been red, the war might have been won.”
Neither have I. The word “if” seems to be quite common for many uses of the subjunctive; while “perhaps” may be correct in some cases I don’t recall ever seeing it used that way.
It seems to me that saying “perhaps the grass were/was red” suggests that it may well have been red, thus would not be a condition contrary to fact (one significant use of the subjunctive) and would call for “was.” Then again, the subjunctive can be used for some cases of possibility, so maybe “were” would be appropriate, but it sounds off to me. I’ll be interested in seeing if someone who knows more than I can clarify that. ETA: Which CookingWithGas has done, at least to some extent.
CookingWithGas: do tenses in different clauses have to agree? “If the grass had been green, I will be a rich man tomorrow.” It sounds dumb, but is it, strictly speaking (ah, Edwin Newman!) forbidden?
I’m not CookingWithGas, but the sentence should read: “If the grass had been green, I would be a rich man tomorrow.”
Subjunctive tense is used not only when the sentence is false but when it is conditional or in doubt. The words “if” or “perhaps” are not necessary to make a sentence subjunctive.
All sentences following “perhaps” are in doubt, but they are not (normally) in the subjunctive (mood not tense), at least not in virtue of being in doubt or following “perhaps”.
The OP’s sentence “Perhaps the grass were red,” is plainly ungrammatical.
Mathematicians use the subjunctive regularly not for contrarity, but for contingency. "A necessary and sufficient condition that a ring have quasi-inverses… But try telling a British copy editor that.
In the interests of full disclosure, I’m CookingWIthGas but I’m not a professional authority in this field. Nonetheless, I will say that moods should be consistent. The first clause is subordinate to the second so if you use subjunctive mood in the first you need conditional in the second, as **barbitu8 **shows. Strictly speaking the tenses don’t have to agree, but they have to be consistent; in that example the tenses/moods are pluperfect subjunctive and future conditional. The error is not in mixing past and future; it is in conflicting moods.
As noted directly above, the subjunctive is not a tense but a mood.
What everyone is talking about here are contrary-to-fact conditionals, and all that really means is using past forms (together with would or could) to indicated that something is a hypothetical, or untrue. The term “subjunctive” is popularly used for this (often even in textbooks), but it’s not truly the subjunctive mood, and I prefer not to do that, to avoid confusion
A true example of the subjunctive would be something like this, indicating a kind of volition:
The judge ordered that Mr. Smith pay five thousand dollars.
In this case, pay is used instead of pays, and in that way demonstrates the subjunctive mood.
It’s important to realize that grammar often serves purposes that are not necessarily representational, so in a discussion such as this it helps not to say past “tense,” but rather just “past form.”
Do you disagree, then, with the Wikipedia article English subjunctive, which describes both the “mandative subjunctive” that you and Hari Seldon are talking about, and the other type, the counterfactual subjunctive that the OP is asking about?
I don’t disagree with the article. (It’s Wikipedia, after all.) I just think the same term shouldn’t be used. Counter-factual conditions shouldn’t be clumped with the others–they’re notionally very different. I’ve said as much in seminars with people like Marianne Celce-Murcia, and they accept my point, and still say that for better or for worse people still use the term that way. But still, when you say something like, “If I hadn’t done that,” etc., you’re not employing a different mood in the same why that the imperative mood is different from the indicative. It’s still a proposition as any other conditional statement is. The fact that it involves counter-factuals doesn’t change that. The Wiki article simply reflects popular habits of terminology.
To continue the point I made just above: The consistency that folks are talking about here is not regarding the mood of the respective clauses, or anything else about “grammar” (i.e., the tenses). It’s about the notional consistency. If I say something like:Have some more waffles, because I really like them.…there’s nothing ungrammatical about the inconsistency of the moods. It’s simply a notional inconsistency. There’s nothing that I’m aware of that requires tenses in and of themselves to be “consistent.”
Can’t wait to see how long it takes before someone pops in with the obligatory comment about the lack of a need for prescriptive grammar. 
(Psst: Read the post directly before yours.)
“I think you should have some more waffles, because I really like them”?