It depends what you want to do with the Venn diagram. The hard part will be detecting the overlapping region, because object tags have priority, and if your mouse is above an intersection, only the topmost (high priority object) will be seen.

If you just want to make visualizations, and not worry about computing whether a point is in an overlapping area, you will probably want to use some sort of transparency. Tk is not very good at transparent objects, but there is an example in the "widget" demo. Tk::Zinc does a better job of transparency, and examples are in the #7 in the OpenGL section of the zinc-demo.

I did notice in the Gnome2-Canvas-1.002 ( the very latest Perl-gtk2 ) demo, that they have transparency. If you run canvas.pl look for the beige oval, and drag it around. You can see through the color transparency. It could be the basis of pretty nice Venn diagram.