where decode seems to do nothing at all

What do you expect it to do?  Is your response content encoded in Latin-2?

the response content is misinterpreted as UTF-8, I think

What exactly is happening that made you arrive at this conclusion?

BTW, you have a literal string "Oldal frissítése..." in your code.  Unless you tell Perl otherwise, it would be interpreted as Latin-1. Is that intended?  Also, you don't seem to have specified an output encoding for the print statements, in particular print $responseText. The latter is decoded content, i.e. a string in Perl's internal unicode format, which should be properly encoded for output.