As an afterthought, to what I said earlier, and the fact that you said this was for a gigantic gui, you might want to worry about memory gains from creating and packForgeting a $mw. PackForget will not destroy the widgets, and repeated use will cause a memory increase the way you have it now.

Some options are to make the $boom a global, and when you call the my_die sub, use $boom over and over, by reconfiguring the buttons and labels. As a matter of fact, if you do it that way, you may not need packForget, just withdraw the $boom and reconfigure the widgets. You only need packForget if you want to repack the widgets in a different order. As an example of a non-leaking reusable popup......

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $tl;
my $mw = MainWindow->new;
$mw->title( "MainWindow" );
$mw->Button( -text => "Toplevel", -command => \&do_Toplevel )->pack();

MainLoop;

sub do_Toplevel {
    if ( !Exists( $tl ) ) {
        $tl = $mw->Toplevel();
        $tl->geometry('300x100+100+100');
        $tl->title( "Toplevel" );
        #put all your widgets here
                
        $tl->Button(
            -text    => "Close",
            -command => sub { $tl->grabRelease;
                              $tl->withdraw }
        )->pack;
    }
    else {
        $tl->deiconify();
        $tl->raise();
        $tl->grabGlobal;
    }
}
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