You didn't provide a Short, Self-Contained, Correct Example so I tossed a bunch of guesses into a pot and stirred...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11118943
use warnings;
use Tk;
use Tk::DialogBox;
use Time::HiRes qw( sleep );

{ # start package
package Tk::AltDialogBox;
use base qw( Tk::DialogBox );
Tk::Widget->Construct('AltDialogBox');
sub Wait
  {
  my $cw = shift;
  $cw->Callback(-showcommand => $cw);
  $cw->waitVariable(\$cw->{'selected_button'});
  $cw->grabRelease if Tk::Exists($cw);
  $cw->Callback(-command => $cw->{'selected_button'}); # swapped with 
+next line
  $cw->withdraw if Tk::Exists($cw);
  }
} # end package

my $message = ' ';
my $mw = MainWindow->new;
$mw->geometry( '200x200+1000+300' );
my $lb;
my $d;
$d = $mw->AltDialogBox( -buttons => ['OK', 'Cancel'],
  -command => sub {
    my $value = $lb->get($lb->curselection // 0);
    $message = "working on $_ ...\n", $d->update, sleep 0.3 for 1 .. 1
+0;
    print "exited by @_ with $value\n";
  },
  -showcommand => sub {
    $lb->selectionClear(0, 'end');
    $lb->selectionSet(0);
    $message = ' ';
  },
  );
$lb = $d->add(Listbox => -height => 10)->pack;
$d->add(Label => -textvariable => \$message, -height => 2)->pack;
$lb->insert('end' => 1 .. 5);
$mw->Button(-text => 'See Dialog', -command => sub { $d->Show },
  )->pack;

MainLoop;
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