I just noticed a very similar Roman numeral golf recently concluded at golf.shinh.org.
Adjusting ySas' winning Perl solution to that game for this one produces this 58 stroker:
which has a similar form to robin
's early-lead 60 stroker above -- though it uses a different algorithm.
Both these solutions are very beautiful, very Perlish, and quite astonishing (at least to me).
I especially enjoyed ySas' ingenious use of @- above. I've never seen @- used like that
in golf before and am tempted to give it a name, "ySas' device".
By way of explanation, note that adding parentheses around (M) above adds one
more element to the @- array when the (parenthesized) M matches.
That is, the number of items in @- is used to
differentiate between a matching M (two items in @-) and
a matching D (one item in @-);
this fits like a glove here because one and two happen to be the required multipliers.
Update: By applying ideas from ySas' solution, we can reduce Robin's
y/DCLXVIM /4 DCLX9/,$\=s/( )?\d/9x($&+!$1)/egfor(<>)x4;print