I'd personally leave it as it is now, keep the permaban.
Plagiarism is just so easy to avoid, in almost all cases it's a deliberate action of the user and should be met with the harshest punishment possible.

Now, I do understand that there might be some cases where the user didn't intend to plagiarize and merely forgot to post a source link.
Those cases are pretty easy to spot though, thus before issuing a permaban, mods should be absolutely certain that the user's intent was to plagiarize.