$_ = unpack("h*", shift); s/([0-9a-f]{2})/$1 /g; print;
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Simpler, if not much shorter ; )

Update: Hmm, this'll work too:

printf("%02x ", $_) for (unpack ("C*", shift));
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2nd Update: I think I'll be trading in my extra whitespaces for a 'chop;'