When to use it
Use it when you're asked about a weakness. Choose a real one that won't disqualify you from the job, and explain what you now do about it. Interviewers have heard “perfectionist” many times; an honest example with a fix is more believable.
Example
Interviewer: “What's a weakness you're working on?”
You: “I used to over-prepare, so now I set time limits and share drafts earlier. It's helped me get feedback sooner.”
In an email or Slack message
Prepare it in writing before the interview: the weakness in one line, what it cost you once, and what you do now. Three lines is enough.