Script-only version:
$set(tracknumber,$left(%_filename%,2))
$set(title,$title($replace($substr(%filename%,3,255),, ))))
Note: the documentation suggests that the $substr function works with (1 or) 2 arguments, but Picard doesn’t allow this: “Wrong number of arguments for $substr: Expected exactly 3, got 2”
Edit: you can also leave out the 3rd argument but keep the comma i.e. only delete ‘255’