Connection to Dark Shadows

I sincerely doubt it.

PM was produced in Hollywood, DS in New York.

I've noticed that a lot of the fancy, stately homes in this show tend to look alike: outer door on the far side (usually the right), with a big stairway next to it and of course in a foyer. THen to the left, across from the outer door, are double doors leading to a study or something. It does tend to look a lot like Collinwood from Dark Shadows, but I think that same general floor plan is used a lot. There are occasional variations with the outer door on the left and the study on the right, but not as often, I don't think. (By the way, similar floorplans are used even much more recently. The Nanny, Fresh Prince of Bel Air, and other sitcoms use similar set designs.)

So instead of Perry Mason having an interior/stairway that looks like Dark Shadows, maybe Dark Shadows simply used some of the same sets used on Perry Mason. 