Here the 1953 RLDS edition changed the first imperfection to the plural. That edition has introduced a plural reading in quite a few cases; for a list, see under 2 Nephi 7:3. Here in Mormon 9:31, either reading will theoretically work, given that later in the passage we have one instance of the singular and another of the plural (“his imperfection … our imperfections”). Note that in this passage we get the singular when the possessive pronoun is singular (mine, his) but the plural when the pronominal form is plural (our). Yet there are no other examples of “ imperfection(s)” in the text, so this relationship is not statistically significant. For another case of the tendency to replace the singular imperfection with the plural, see below under Mormon 9:33. Here in verse 31, the critical text will follow the earliest reading, thus “mine imperfection … his imperfection … our imperfections”.
Summary: Maintain in Mormon 9:31 the singular imperfection when preceded by mine and his, but the plural imperfections when preceded by our.