save module as utf8(or any other enconding from list above) file, from your error description it is ascii
I'm pretty sure I'm saving it in unicode. I can use all unicode chars easily in string literals ("x²") and output them to console. But using them in identifiers leads to compiler error.
Apart from that, try this code:
int в_квадрате(int num) { return num*num; }
writeln(2.в_квадрате);
You get: Error: found 'в_квадрате' when expecting ','
