When I tried to uninstall Tk, I received a message that TK was not found. It seems like my attempt to uninstall Tk::Canvas had uninstalled all of Tk. When I tried to reinstall Tk, Norton Anti-Virus found viruses in at least two files, including "Win64.Evo.genTrj" in Canvas.xs.dll, which of course caused the install to fail. I now suspect that this whole chain of events was started by Norton downloading a new virus signature which fit this file. It must have removed or quarantined the file without my noticing it. I strongly believe that this is a false positive, but I have not decided how to proceed.