In this verse, there is some variation, especially when compared with the corresponding language in the book of Genesis:
The first question to consider is whether the original text for Alma 12:23 read “if thou eatest” (the King James text has “that thou eatest”) rather than “if thou eat” (the Book of Mormon reading). Here in Alma 12:23, the verb eat takes the subjunctive form without the inflectional ending -est. The original text has examples of subjunctive verb forms in subordinate clauses, as in the following examples:
Thus the occurrence of “if thou eat” as the earliest reading for Alma 12:23 is quite possible. Although the original manuscript is not extant for thou eat, spacing between surviving fragments indicates that eat fits better than eatest. The critical text will therefore maintain the earliest reading, “if thou eat”, in Alma 12:23.
The second case of variation here in Alma 12:23 has to do with the thereof that scribe 2 of 𝓟 originally wrote. Oliver Cowdery, when proofing 𝓟, crossed out this thereof. Based on spacing between extant fragments, there is no room in 𝓞 for a thereof except by supralinear insertion. Scribe 2’s thereof was probably the result of his familiarity with the biblical account of Adam and Eve in the Garden of Eden. Interestingly, there are 24 occurrences of “eat thereof” in the King James Bible, but none in the Book of Mormon text; so Oliver Cowdery’s crossing out of the thereof is entirely consistent with the language of the Book of Mormon. The critical text will therefore accept the reading without the thereof.
Finally, the 1837 edition replaced the shalt with shall in the phrase “thou shalt surely die”. This change appears to be accidental. For discussion of the phraseology thou shall (which is occasionally found in the earliest text of the Book of Mormon), see under Mosiah 12:11. In this instance, however, the earliest text reads thou shalt, in agreement with the corresponding King James passage. The 1852 LDS edition restored the original shalt to the LDS text, but the RLDS text has retained the 1837 thou shall.
Summary: Maintain in Alma 12:23 the corrected reading in 𝓟, “if thou eat / thou shalt surely die”; spacing between extant fragments of 𝓞 argue for this shorter reading rather than a longer reading based on the King James text (“if thou eatest thereof / thou shalt surely die”).