Show us some code. The following updates the label in the main window before opening the dialog.

use strict;
use warnings;
use Tk;
use Tk::Dialog;

my $mw = MainWindow->new;
my $label = $mw->Label (-text => "Foo bar bang")->pack;
my $button = $mw->Button (-text => "Bar bang foo", -command => \&popup
+)->pack;

MainLoop;


sub popup {
    my $dialog = $mw->Dialog;
    
    $label->configure (-text => "Bar bang foo");
    $dialog->Show ();
}
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