You cannot use anchor popup to do that.
But there are work around, for example you can draw the dialog box like this at 50x50 screen coord*:
use Tk;
$mw = MainWindow -> new;
$openbutton = $mw -> Button (
-text => 'Open',
-command => \&opensub,
)-> pack;
sub opensub {
$top = $mw ->Toplevel;
$top -> wm('geometry', '0x0+50+50');
$top->overrideredirect(1);
$type_file = [['GIF Image', '.gif'], ['JPG Image', '.jpg']];
$file_openned = $top->getOpenFile(-filetypes => $type_file,
-title => 'Choose an image',
-defaultextension => \$type_file);
}
MainLoop;
Hope this help.