G'day J-Monk,

Welcome to the monastery.

From the rules you've supplied, "/INBOX is migrated to INBOX.INBOX" is absolutely correct.

There are no supplied rules where "/INBOX" would become "INBOX".

There is either something wrong or missing in what you've posted. Please fix whatever that is so we can help you. (You might find "How do I change/delete my post?" helpful with this.)

One hint. Your blanket use of the 'g' modifier in your regex substitutions suggests you don't really understand what this is for. See "perlre: Modifiers".