When copying from 𝓞 to 𝓟, Oliver Cowdery accidentally added the modal verb may before perish. The 1852 LDS edition restored the reading of the original text by removing the may, although this change was not done by reference to the original manuscript (which at the time was still lying in the cornerstone of the Nauvoo House). The 1852 editors probably deleted the may because they recognized the possible contradiction between “may perish” and “are lost”: the modal expression “may perish” implies that these deceived people might not actually perish, but the following predicate “are lost” definitely implies that they are indeed lost. One could argue that a lost soul can be found, but that if one has perished, there is no hope of recovery. A similar passage in Alma, however, indicates that being lost and perishing are essentially the same from a spiritual point of view:
The use of the modal must here in Alma 34:9 clearly points out that there is no other option. The intrusive may in 1 Nephi 12:17 is definitely inappropriate, and thus the 1852 LDS editors were justified in removing it. Of course, we now know that their emendation is in fact the original reading. The RLDS text has, however, continued with the intrusive may.
Summary: Maintain the reading of the original manuscript in 1 Nephi 12:17 (“that they perish and are lost”); the 1852 edition restored the original reading, probably because the editors noticed the inconsistency of “they may perish and are lost”.