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

use Tk;
my $mw = tkinit;
my $labelvar = 0;

$mw->Label(-textvariable=>\$labelvar )->pack;
$mw->Button( -text => "Button", -command => sub {
    $mw->Busy;
    for( 1 .. 10){
        $labelvar=$_;
        $mw->update;
        select(undef, undef, undef, 0.25);#sleep of 250 milliseconds
    }
    $mw->Unbusy;
})->pack;

$mw->MainLoop;