According to How to display lines to tk text box in real time the update should work. Here's the counter generator from that link

use strict;
use Tk;
use Tk::ROText;

my $main = MainWindow->new;
my $b = $main->Button(-text => 'count', -command => \&count )->pack;
my $t = $main->ROText(width => 80, height => 10) ->pack();
MainLoop;
sub count {
   my $i = 0;
   my $count;
   while ($i<20) {
     $t->insert("end", "Line $i\n");
     $t->update();
     sleep 1;
     $i++;
   }
}
[download]

Works on my "This is perl, v5.8.0 built for i386-linux-thread-multi" and "Red Hat Linux release 9 (Shrike)"

Not sure if a process would change that - cant see why, your in the loop with a new value for $_, so should just work. I'll try here to with something like 'cat $0' so I place my own code in the tk window - ok, done - worked fine. Can you give anymore info ? Does the process return any blank lines - they could be hard to see :P