In file included from src/mappedio.c:100:
src/streamio.h:204:15: error: expected identifier or '(' before numeri
+c constant
 #define CR    0xD
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For thoroughly sane and understandable reasons that I could not fathom, the one occurrence of CR in streamio.h needs to be changed to something else. (I chose CROWBAR)
Then, of course, the 2 occurrences of CR in streamio.c, need to be changed to whatever symbol was used in streamio.h (ie for me, CROWBAR) .

So:
1) Locate your home_directory\.cpan\build\Alien-Tidyp-v1.4.7-0\build_src\tidy-1.04\src\streamio.h and home_directory\.cpan\build\Alien-Tidyp-v1.4.7-0\build_src\tidy-1.04\src\streamio.c files, and make the changes as stated above.
2) Then cd to your home_directory\.cpan\build\Alien-Tidyp-v1.4.7-0 folder and run perl build. This will prompt you as to whether to use freshly unpacked source or not.
Make sure you choose "No" (which is the default). If you choose yes, then you'll only have to make the same changes again.
3) Then run perl build test, and if that succeeds (as happened for me on Windows 7), run perl build install.
4) Then you should be set to run cpan -i HTML::Tidy again.

Cheers,
Rob