Assuming your Tk::WaitBox is a toplevel widget (which it probably is)....

my $screenheight = $mw->screenheight;
my $screenwidth  = $mw->screenwidth;

# Force it on the screen and to take a size if it
# hasn't already
$waitbox->update(); 

my $wbwidth = $waitbox->width();
my $wbheight = $waitbox->height();
my $x = ($screenwidth - $wbwidth) / 2;
my $y = $screenheight - $wbheight;
$waitbox->geometry("+$x+$y");
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