Getting Perl/Tk running under cygwin seems to be a constant pain. Until the recent perl release which went to a site/5.8 directory for modules, this was something that I went through every time I upgraded perl. I think I found the secret though. Apparently they dropped cygwin support in Tk804. You need to get Tk800.025 from cpan. Make test still reports a few errors, but the installation works for me.