From what I have read, there is a long debate about how much the slavs and those from scandinavia influenced each other.

As to Perun and Thor, it seems to me that most Germanic tribal groups had a god that held similar stature. I could not say based on what I have read which came first, and I suspect its a little like the chicken and the egg argument.

My gut feeling tells me that they likely both grew over similar time periods and influenced each other.

Recall that it is not the Rus that the slavs first meet from that area but likely the Goths as they moved south in from 300 BC on before invading the Roman empire. So there is a lot of touch with this mythology from a long time ago I think.

What I would wonder is you have a buffer group between them both, the Urgic (spelling?) peoples, and I would be curious how their view of a Perun/Thor like god would be.