Thanks for the help, ikegami. utf8::downgrade had the same effect.
ADO can be set up to accept utf8 through Win32::OLE. I had thought that my script already was set up like that, but when I looked at where it was declared, I saw that it was done in some old, probably deprecated way. Setting Win32::OLE->Option( CP => Win32::OLE::CP_UTF8 ) seems to solve all my problems. Thanks again!