Well what do you know, I had Tk (the Tcl extension) lying around installed from Fink (which is really weird since I switched to darwinmacports a while ago . . .). Clobbered that one and what do you know there's a /usr/bin/wish and what looks like corresponding libraries and includes under /usr/{include,lib}. Make sure you don't have any other copies of Tk (the Tcl extension) lying around and then you should be able to reinstall Tk (the Perl module) and it should compile against the native Cocoa/Carbon-y copy.

(And thanks for pointing that out merlyn. I didn't know that shipped with OS X :)