As said above, Thor and Perun are deities of two related IE nations. My personal believe is that it is the same deity known under different names. The name of the Thundermaster vary from nation to nation: Amon, Marduk, Indra, Virakocha, Zeus, Jupiter etc. The Thundermaster stands as a "gray eminence" behind the evolution of white indoeuropeans the same way as f.e. god of mars stands behind the mongolic race, just remember the greek myth about Zeus and Europa. This paternal supervision of Perun is also the reason why he was so worshiped among Slavs and other IE nations. Not the supreme deity, but among the highest.