I would very strongly suggest that the user should expect Rmpz_import() to process the series of base-256 "digits" obtained by (map ord($_), split //, $str), regardless of the internal encoding of the string. So (a) is the correct result. (b) is just horrible, and is repeating the broken Unicode model that appeared in perl 5.6 and was (mostly) fixed by perl 5.8.

Your only real decision needs to be what to do for a codepoint > 0xff. Three obvious choices are: croak; treat each codepoint modulo 256, or carry the overflow into the next digit. So the string "\x40\x{150}\x60" would yield the integer value 0x615040. (I haven't looked at what endedness the function works to, but that should give you the general idea of what I mean.)

Dave.