Ok, here's how I would do it the other way: Make a lookup table of the translations (since it's not straightforward base conversion):

$_="MO\x81B\x8dCAJ\xa3";
my $start = 0x80;
my %xlate = map { my $first = $_;
                  map {(chr($start++), "$first$_")} (0..9, '.')
                } (0..9,'.') ;
s/([\x80-\xec])/$xlate{$1}/g;
print;
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The compound map builds the translation table.