Here's one way to do it:

#!perl -w
use strict;
my @ddig;
my @spaces;
$ddig[$_ + 128] = sprintf "%02d", $_ for 0 .. 99;
$spaces[$_] = " " x $_ for 0 .. 255;


while (<DATA>) {
  s/([\x80-\xe3])/$ddig[ord($1)]/g;
  s/\xff(.)/$spaces[ord($1)]/eg;
  print;
}
__DATA__
MO�B�CAJ�
This is a�test
123456789012345678901234567890
[download]

Please note, as Roy Johnson says above, that either your specification is unclear or your arithmetic is wrong. I'm assuming that "\x00-\xe3" maps to "00" through "99". The example you gives maps to "MO01B13CAJ35" with my code, not "MO01B12CAJ32".