Thanks for the comment. The colleague originally working on this function already posted a question regarding the possibility to call a Perl subroutine from $int->Eval(); on the tcltk@perl.org group. It seems it is possible to run Perl code from within $int->Eval(); with the Tcl command ::perl::Eval, however not a Perl subroutine that lives outside the $int->Eval();. Since I have 0 knowledge of Tcl and no experience in "language bridging", I was hoping to solve the issue simply by monitoring the change in the Listbox state/values. There are a couple of suggestion in other answers here that I am now examining.