I watched documentary, and I'm aware of that Perun/Veles toponomy, but still it isn't some big proof about some great struggle between them. I'm curious about this, since some scholars aprove this theory and some other don't.
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.
But St. George (Juraj, Ђорђе) was also equated with Jarilo and Triglav.
"Прошета се свети Ђурђе,
Наш вељи Триглаве,
Низ хубаво равно поље"
(Милојевић, 1875: бр. 198).
Also one of the main atributes of god Veles was bear.
Maybe eastern Slavic folklore would provide us more information about this Perun/Veles subject, but I couldn't find it.
Veles, by N. Aleš