Body protection warranty
The perforation must not have been caused originally by damage, neglect, insufficient care or maintenance or by external rusting.
I had a similar issue with a 2year old Mazda 6 (54 plate). Rust on all 4 wheel arches.
Searching the internet and talking to the bodyshop tech it turns out Mazda used cheep metal for a few years as there was a shortage and a lot of cars were now displaying rust around the wheel arches.
The set up I had to go through was (which I'd imagine would be similar to VW), take the car to the garage for pictures to be taken, these were then emailed off to Mazda head office, waited several months for an answer on whether they would repair them or not.
Here's the bad news, they agreed to repair the rear wheel arches as these had corroded from the inside out due to poor metal. The fronts that looked exactly the same to me were corroding from the outside in and not covered from their anti perforation warranty (external rusting). I took it further and demanded some big shot come from head office. They still refused to budge. He claimed it was completely different.
I think the fact the cost for repairing the 2 rear wheel arches that they were covering, £2800 compared to the front which they refused to do at only a couple of hundred pounds said to me they were probably right.
Its an even longer story in that the garage did the work, told me they changed all the rear panels etc only for the rust to return 6 months later. Bar-stewards had only rubbed the rust down and painted it!!! Claimed Mazda instructed them to do this first!! Whole scenario took about 2 years (I kid you not) and in the end I got that fed up I traded the car in for the golf. They still never changed the rear panels and to this day I'm still waiting on Mazda contacting me!!!