Unpack has the little known property to be able to calculate sums in several bit widths. In this case first convert the hexadecimal representation to a character string, sum over that in 8 bits and invert: 
perl -wle 'print 0xff ^ unpack "%8C*", pack"H*", "00018108000202020202
+0202"'
103

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In reply to Re: XOR'ing to calculate a hex checksum by thospel
in thread XOR'ing to calculate a hex checksum by c4onastick

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