You can keep a reference to the label and update it directly. This is generally how you have to handle any progress/meter type stuff that needs to be updated in a loop. (This is untested but should be pretty close)

my $counter_label = $mw->Label(-text => $no,
     -relief => 'sunken', 
    -anchor => 'center')
    ->place(-relx =>0.0, -rely => 0.80, -relwidth => 1.0, -relheight =
+> 0.2);

...

sub count {
    if ($upper_no > 0) {
        $no = 0;
        while ($no < $upper_no) {
            $no++;
            $counter_label->configure(-text => $no);
        } 
    }
}
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