Unfortunately drive letters are not an option.

Just to make sure that it is really the UNC path which causes griefs: Could you at least try in a test run to use drive letters instead, so that you can confirm the cause?

I don't know what Tk is doing internally so that your call fails. UNC pathes are OK in *most* cases, but here are two exceptions I know, which don't work with UNC pathes: If you use the Cygwin version of Perl, or if you set *on the command line* the "default drive" to a UNC path (which doesn't seem to apply in your case). But of course there might be other issues I'm not aware of.