If you read "perldoc Tk::Dialog" it states that all positioning is done by the Popup method, read "perldoc Tk::Popup". Popup positions are limited. If you want to move your dialog, you need to roll your own, with a toplevel window. It's not that hard, you just add a global grab.

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

my $tl;
my $mw = MainWindow->new;
$mw->title( "MainWindow" );
$mw->Button( -text => "Toplevel", -command => \&do_Toplevel )->pack();

MainLoop;

sub do_Toplevel {
    if ( !Exists( $tl ) ) {
        $tl = $mw->Toplevel();
        $tl->geometry('300x100+100+100');
        $tl->title( "Toplevel" );
        #put all your widgets here
                
        $tl->Button(
            -text    => "Close",
            -command => sub { $tl->grabRelease;
                              $tl->withdraw }
        )->pack;
    }
    else {
        $tl->deiconify();
        $tl->raise();
        $tl->grabGlobal;
    }
}
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