the problem is identified and resolved in Tk 5.8.9
This means that if you're using ActivePerl you need to use and update of their "tkkit.dll", which is not available right now.

read this explanation at http://www.mail-archive.com/tcltk@perl.org/msg00512.html

For me, I can't reproduce the problem, but I do not use Hiragana input either.
I do use pinyin input sometimes, and - just tried - it is working in all my Tk versions available