But it looks that isn't the case…watch the documentary, it gets really detailed about that Veles vs Perun part and how the (at least here in Croatia) early Croats used to name mountain a Perun, the between there would be a river meaning the cross between the two worlds, and on the other side of the river it would be a plain usually topology named by Veles.
I will look for sources in one book i have at home. There's a lot of folklore song mentioning it.
Here's one Croatian early-medieval folk song that mentions Perun (sv.Juraj) killing Veles (dragon):
"V tom časě drakun iz jezera se isklonjaše.
Sveti Juraj ga zagledaše.
Znamenijem svetago križa on se znamenaše,
šćita i sulice rukama potresnjaše,
tr drakuna v grlo probodjaše."
After the christianization "Sveti Juraj" and "Sveti Ilija" take Peruns attributes.