A very creative solution :)
Be sure to check out Roman if you didn't know it already. It can convert roman numbers both ways, and checks if your roman input is valid.

use Roman;
$calc = "II + II";
$calc =~ s/([MDCLXVI]+)/isroman($1) ? arabic($1) : $1/;
print eval $calc;
[download]

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
[download]