If I justify myself, mine own mouth shall condemn me. Since he could not wholly justify himself. "All men have sinned, and come short of the glory of God" (Romans 3:23). Job has already admitted the utterance of "rash words" (Job 6:3), and, at least hypothetically, that he "has sinned" (Job 7:20), and needs "pardon" for his "transgression" (Job 7:1-21:24).
Job, if he tried to "justify himself," would have to acknowledge such shortcomings, such imperfections, such sins—at any rate, of infirmity—as would make his attempted justification a real self-condemnation.
If I say, I am perfect, it shall also prove me perverse; rather, even were I perfect, it (i.e. my mouth) would prove me perverse; i.e. supposing I were actually perfect, and tried to prove it, my speech would be so hesitating and confused, that I should only seem to be perverse.