OK, consider the following example. The windows are exactly the same size, yet appear in different positions. What do I need to to do the 2nd popup to get it to be centered in the main window?

use strict;
use warnings;
use Tk;

my $mw = MainWindow -> new;
$mw -> minsize (qw(600 400));

my $button = $mw -> Button (
                    -text => '1st popop',
                    -command => \&popup1,
                    ) -> pack;

my $other_button = $mw -> Button (
                    -text => '2nd popup',
                    -command => \&popup2,
                    ) -> pack;


MainLoop;

sub popup1 {
    print "Main window geometry = ".$mw -> geometry."\n";
    my $popup1 = $mw -> Toplevel;
    $popup1 -> withdraw;
    
    for (1..10) {
        my $label = $popup1 -> Label (
                    -text => "This is a very very very long label",
                    ) -> pack;
    }

    $popup1 -> Popup (
                -popover => $mw,
                -overanchor => 'c',
                -popanchor => 'c',
                );
                
    print "1st popup geom = ".$popup1 -> geometry."\n\n";
}

sub popup2 {
    print "Main window geometry = ".$mw -> geometry."\n";
    my $popup2 = $mw -> Toplevel;
    $popup2 -> withdraw;

    my $label = $popup2 -> Label (
                    -text => "This is a label",
                    ) -> pack;
                    
    $popup2 -> minsize (qw(162 190));
    
    $popup2 -> Popup (
                -popover => $mw,
                -overanchor => 'c',
                -popanchor => 'c',
                );
                
    print "2nd popup geom = ".$popup2 -> geometry."\n\n";
}
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