Add $^E to your error report and you might get a better idea what is going wrong.

My only guess is that your script is running in a context where it doesn't have a current "working drive" and if you prepend the "C:" to your string, that it might fix the problem.

You can see the code that is producing this error at p5git://win32/win32.c., in particular:

    dirp->handle = FindFirstFileW(PerlDir_mapW(wscanname), &wFindData)
+;

    if (dirp->handle == INVALID_HANDLE_VALUE) {
        DWORD err = GetLastError();
        /* FindFirstFile() fails on empty drives! */
        switch (err) {
        case ERROR_FILE_NOT_FOUND:
            return dirp;
        case ERROR_NO_MORE_FILES:
        case ERROR_PATH_NOT_FOUND:
            errno = ENOENT;
            break;
        case ERROR_NOT_ENOUGH_MEMORY:
            errno = ENOMEM;
            break;
        default:
            errno = EINVAL;
            break;
        }
        Safefree(dirp);
        return NULL;
    }
[download]

So EINVAL (invalid argument) just means "not ERROR_NO_MORE_FILES, ERROR_PATH_NOT_FOUND, nor ERROR_NOT_ENOUGH_MEMORY". $^E should tell you what GetLastError() returned.