Argh. Meant to simplify by decrementing the last byte and explicity setting the the next to last byte to 255

sub prepchksum {
    my ($payload) = @_;
    printf "BEFORE = %s\n", (CORE::unpack "H*", $payload);
    if (length($payload)%2) {
      $payload =~ s#(.)$#\0$1#;
      printf "MID1   = %s\n", (CORE::unpack "H*", $payload);
      # Compensate off-by-one error
      if (ord(substr $payload, -1) >= 128) {
         $payload=~s#(.)(.)$#pack('CC',255,
             (ord($2)-1)&255)#e;
      }
   }
   printf "AFTER  = %s\n", (CORE::unpack "H*", $payload);
}
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