Now, using Tk804, it just freezes, and refuses to work. Does anyone have any idea why?

Because its a bad idea (should've never worked)? All MainLoop consists of is

sub MainLoop
{
 unless ($inMainLoop)
  {
   local $inMainLoop = 1;
   while (Tk::MainWindow->Count)
    {
     DoOneEvent(0);
    }
  }
}
[download]

So just DoOneEvent(0) a couple of times, and then return the progressbar. Example

use Tk;
my $pb = launch_progress();

for(1 .. 100)
{
    $pb->value($_);
    DoOneEvent(0);
    select undef, undef, undef, 0.06;
    warn $_,$/;
}

undef $pb;
exit;

sub launch_progress {        
        my $main_window = Tk::MainWindow->new;
        my $tk_progressbar = $main_window->ProgressBar(
              -width => 20,
              -height => 300,
              -from => 0,
              -to => 100,
              )->pack( -fill=>"x", -pady => 24, -padx => 8 );


        my $c = 0;
        while (Tk::MainWindow->Count and $c < 10) {
            DoOneEvent(0);
            $c++;
        }

        return $tk_progressbar;
}
[download]