Just use the -global option for the Show method of a dialog:

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

use Tk;

my $mw    = 'MainWindow'->new(-title => 'mw');
my $tl1   = $mw->Toplevel(-title => 'tl1');
my $tl2   = $mw->Toplevel(-title => 'tl2');
my $modal = $tl2->Dialog( -title => '2/dialog');
$modal->Show(-global);
MainLoop();
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