I've did a nasty hack and now it works. It doesn't crack, it doesn't hang but it ain't prety.
I had put the code here for educational purposes only :).

#!/usr/bin/perl
use Tk; 
use strict; 
require Tk::ProgressBar; 
require Tk::Button; 
my ( 
    $mw, 
    $progressBar, 
    $cancelCalButton, 
    $runCalButton, 
   ); 
my $guiPid = $$; 
$mw = MainWindow->new(); 
$cancelCalButton = $mw->Button(-text=>"cancel")->pack(); 
$runCalButton = $mw->Button(-text=>"run",-command=>\&doit)->pack(); 
$progressBar = $mw->ProgressBar()->pack(); 
 my $val = 0;
$SIG{USR1} = sub { 
  $val = 0;
  $progressBar->value(0); 
  $mw->update(); 
  $runCalButton->configure(-state=>"normal"); 
  $cancelCalButton->configure(-state=>"disabled"); 
}; 
$SIG{USR2} = sub { 
    $val++;
    $progressBar->value($val*10); 
    $mw->update(); 
}; 
MainLoop; 
sub doit { 
  my $calPid; 
  $runCalButton->configure(-state=>"disabled"); 
  $cancelCalButton->configure(-state=>"normal"); 
  $mw->update; 
  print "Failed to fork" if (!defined($calPid = fork())); 
  return if ($calPid); 
  for (1 .. 10) { 
    $progressBar->value($_); 
    kill('USR2',$guiPid); 
    sleep(1) ;
  } 
  # send the signal to main GUI to reconfigure the button states 
  kill('USR1',$guiPid); 
  kill('INT',$$); 
}
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