#!/usr/bin/perl
use strict;
use Tk;
use Tk::ProgressBar;

# make window
my $mw = new MainWindow();

# tell MainWindow to execute this function after 1000 milliseconds
$mw->after(1000,\&MyUpdate);

# or tell MainWindow to execute this sub repeatedly
$mw->repeat(1000,\&MyOtherUpdate);

MainLoop;

sub MyUpdate {
    # ... start something?
}

sub MyOtherUpdate {
    # ... update window, reset progress bar?
}
[download]

Untested!

PS If you use the progress bar, you must remember to update the window (via $mw->update())