Compiled eRuby 1.0.5 under MinGW:

$ /c/mingw/usr/local/bin/eruby --version
eRuby version 1.0.5
ruby 1.8.2 (2004-09-06) [i386-mingw32]
$ _

Here the truncation problem does not occur.  Still, I wonder why this
all happened with the mswin32 build of ruby/eruby.

But at least I can now continue development at home at a Win32 box.