Revered Monks,
It is with humility and penitence that I bring to you my first question. Please forgive my ungraceful coding style and lengthy post, as I am new to this monastery, and am but a lowly electrical engineering student.

I am an engineering intern for the summer, and have been made by my superiors to learn to code in Perl. Among other things, I have to program a GUI that allows the engineers to create a list of solutions to a problem from an established list. They also have to be able to edit/remove any one of these solutions from the same interface.

The issue I'm having is as follows: I (plan to) load the available solutions from a text file into $AvailListbox, from which they can be moved to and from $SelectedListbox using two buttons. If a solution needs to be removed or modified, I have a button that opens a Toplevel into which (for convenience) I pack the already existing $AvailListbox.

When I withdraw the Toplevel, however, $AvailListbox is still present in $MW, but can no longer be accessed as an element thereof. In other words, any subroutine I try to call using $AvailListbox->curselection() does not work on the selection in $MW, but rather in the Toplevel, even after it has been withdrawn. I'm wondering how I would go about restoring the "control" over $AvailListbox to $MW. And hopefully that makes sense.

Here are some relevant sections, with most of the formatting calls in pack() or widget creations removed, for readability. If you paste all the snippets of code together, it should reproduce the same scenario as I mention above.

Here I create $AvailListbox, which holds the available solutions.

#!/usr/local/bin/perl

use warnings;
use strict;
use Tk;

# Predeclaring the subroutines and variables
sub initialize_step_selection();
sub select_soln_step();
sub deselect_soln_step();
sub modify_remove_step_dialog();

# Building the GUI skeleton, i.e. predeclaring some widgets
my $MW = MainWindow->new;

my $StepFrame;
my $AvailListbox;
my $SelectListbox;

# Run the subroutines
initialize_step_selection();

MainLoop();

# Creates the solutions frame, and its contents
sub initialize_step_selection() {
    
    # Frame for selecting solutions
    $StepFrame = $MW->Frame();
    $StepFrame->pack(-fill    =>    'both');
    
    # Scrollbars for available solutions
    my $AvailScroll = $StepFrame->Scrollbar(-orient    =>    'vertical
+');
        
    # Listbox of available solutions
    $AvailListbox = $StepFrame->Listbox(-yscrollcommand    =>    ['set
+'    => $AvailScroll]);
    $AvailListbox->pack(-fill    =>    'y', -side    =>    'left');
    $AvailScroll->configure(-command    =>    ['yview'    => $AvailLis
+tbox]);
    $AvailScroll->pack(-fill    =>    'y',-side    =>    'left');
    $AvailListbox->insert('end', "Just", "for", "testing", "purposes")
+;

    # Scrollbar for selected solutions
    my $SelectScroll = $StepFrame->Scrollbar(-orient    =>    'vertica
+l');
        
    # Listbox of selected solutions
    $SelectListbox = $StepFrame->Listbox(-yscrollcommand    =>    ['se
+t'    => $SelectScroll]);
    $SelectListbox->pack(-fill    =>    'y', -side    =>    'left');
    $SelectScroll->configure(-command    =>    ['yview'    => $SelectL
+istbox]);
    $SelectScroll->pack(-fill    =>    'y',-side    =>    'left');

    # Select/Deselect solutions buttons
    my $SelectStep = $StepFrame->Button(-text    =>    ">",-command   
+ =>    sub{select_soln_step();});
    $SelectStep->pack();
    
    my $DeselectStep = $StepFrame->Button(-text    =>    "<",-command 
+   =>    sub{deselect_soln_step();});
    $DeselectStep->pack();
    
    my $ModRmvStep = $StepFrame->Button(
        -text                =>    "Modify/Remove a Step",
        -command            =>    sub{modify_remove_step_dialog();}
    );
    $ModRmvStep->pack(
        -padx                =>    2,
        -pady                =>    3);

} # Closing initialize_step_selection()
[download]

And then I repack $AvailListbox in the Toplevel here:

sub modify_remove_step_dialog() {

    my $ModRmvStep = $MW->Toplevel();
    $ModRmvStep->title("Modifier une étape");
    
    my $MRSFrame1 = $ModRmvStep->Frame(-label    =>    "Sélectionnez u
+ne étape à modifier");
    $MRSFrame1->pack(-fill    =>    'both');
    
    # Listbox of available solutions
    my $StepModRmvScroll = $MRSFrame1->Scrollbar(-orient    =>    'ver
+tical');
    $AvailListbox = $MRSFrame1->Listbox(-yscrollcommand    =>    ['set
+'    => $StepModRmvScroll]);
    $AvailListbox->pack(-fill    =>    'both',    -side    =>    'left
+');
    $AvailListbox->insert('end', "Just", "for", "testing", "purposes")
+;
    
    # Configuring available listbox scrollbars
    $StepModRmvScroll->configure(-command    =>    ['yview'    => $Ava
+ilListbox]);
    $StepModRmvScroll->pack(-fill    =>    'y', -side    =>    'left')
+;
    
    my $MRSFrame2 = $ModRmvStep->Frame();
    $MRSFrame2->pack(-fill    =>    'both');
    
    my $MRSMod = $MRSFrame2->Button(-text    =>    'Rien');
    $MRSMod->pack();
    
    my $MRSCancel = $MRSFrame2->Button(-text    =>    'Rien');
    $MRSCancel->pack();
        
    my $MRSRmv = $MRSFrame2->Button(-text    =>    'Cancel', -command 
+   =>    sub{$ModRmvStep->withdraw;});
    $MRSRmv->pack();

}
[download]

Now, when I call sub select_soln_step() by pushing the top button in the $MW, it moves the selected solution from the left box to the right one. The bottom button does the opposite:

# This subroutine will move the selected solution from the AvailListbo
+x
# to the SelectListbox
sub select_soln_step() {

    # Get the contents of the current selection    
    my $selIndex = $AvailListbox->curselection();
    
    # If no item is selected, we do nothing.
    unless($selIndex eq "") {
        my $selection = $AvailListbox->get($selIndex);
    
        # Remove the selected value from the AvailListbox, and add it 
+to the SelectListbox
        $SelectListbox->insert('end', $selection);
        $AvailListbox->delete($selIndex);
    }
    
}


# This subroutine will move the selected solution from the SelectListb
+ox
# to the AvailListbox
sub deselect_soln_step() {

    # Get the contents of the current selection
    my $selIndex = $SelectListbox->curselection();
    
    # If no item is selected, we do nothing.
    unless($selIndex eq "") {
        my $selection = $SelectListbox->get($selIndex);
    
        # Remove the selected value from the AvailListbox, and add it 
+to the SelectListbox
        $AvailListbox->insert('end', $selection);
        $SelectListbox->delete($selIndex);
    }
}
[download]

However, once the Toplevel has been opened, the button to move the solutions from the left box to the right one no longer works on the curselection(); in $MW, but on the curselection(); in the Toplevel... Is there any way to give control over $AvailListbox to $MW?