use strict;
use warnings;

use Tk;

my $mw = MainWindow->new();

my $single_tl;

$mw->Button(-text => 'Open Toplevel',
    -command => sub {
        if (Exists($single_tl)) {
            $mw->messageBox(-message => 'Already Open');
        }
        else {
            $single_tl = $mw->Toplevel();
            $single_tl->Button(-text => 'Close',
                -command => sub { $single_tl->destroy(); },
            )->pack();
        }
    },
)->pack();

$mw->Button(-text => 'Exit', -command => sub { exit })->pack();

MainLoop;

##</code><code>##

use strict;
use warnings;

use Tk;

my $mw = MainWindow->new();

$mw->Button(-text => 'Open Toplevel',
    -command => sub {
        my $single_tl = $mw->Toplevel();
        $single_tl->Button(-text => 'Close',
            -command => sub { $single_tl->destroy(); },
        )->pack();
        $single_tl->grab();
    },
)->pack();

$mw->Button(-text => 'Exit', -command => sub { exit })->pack();

MainLoop;