unpack primarily deals with bytes, and H is no exception. Getting the hex representation of the byte "\x{2020}" makes no sense.

ord gets the UNICODE codepoint and sprintf can convert to hex.

sprintf('%x', ord("\x{2020}"))
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You can do some nifty stuff with Encode, too.

encode('US-ASCII', "ab\x{2020}cd", Encode::FB_PERLQQ)   # ab\x{2020}cd
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See other values for the third param.