I know this will involve some sort of bind + callback, but I cannot figure out how to do it...

You can always look it up :) site:perlmonks.org Tk "bind(" text entry -> Tk::Entry selection problem

You can also find examples with

$ perl -MTk -le " print Tk->findINC() "
C:/perl/site/5.14.1/lib/MSWin32-x86-multi-thread/Tk

$ ack --match "->bind" C:/perl/site/5.14.1/lib/MSWin32-x86-multi-threa
+d/Tk
...
[download]