// get function argument
char* NodeName = (char *)SvPV(ST(0),PL_na);

// get argument length
int nNodeNameLen = strlen(NodeName) + 1;

// alloc memory
LPWSTR wNodeName = (LPWSTR)malloc(nNodeNameLen * sizeof(WCHAR));

// do conversion
mbstowcs(wNodeName, NodeName, nNodeNameLen);

// Here's where I'd actually use the data
// I'm leaving out the part where I allocate widebuffer
someLibFunction(wNodeName, widebuffer, buffersize);

// de-allocate
free(wNodeName);

/*
    And now I'd have to do it all in reverse: 
    allocate a new char* buffer, convert the output
    back to char*, deallocate the wide buffer, put 
    the result back on the stack, and free the wide buffer.
*/