The same way you can't really push film to any great degree, you can't pull it to any great degree. If you simply underdevelop a lot you will lose too much contrast. A speed decreasing developer might give you about a stop less speed, but I would not use Perceptol to do this because you're going to have a grainy negative in any case, and Perceptol will just make it mushy grain.
Whatever developer you use, you might try reversing the procedures we normally when we try to maximize speed and reduce contrast. Dilution of the developer and intermittent agitation with a longer development time helps maximize speed for a given contrast level. So reversing this logic, you might consider a more concentrated dilution, with a shorter development time and continuous agitation. Testing would be required to find the right time so if this roll is really important do some testing on spare rolls first.
In any case you'll still end up with a dense negative. The goal here is to try to get fairly normal contrast, and hopefully highlight detail assuming the highlights were not exposed beyond the scale of the film.