If you want to use Tk, you can use the fileevent method, which will sit there and wait until it sees output.

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

my $mw = new MainWindow;

my $t = $mw->Text(-width => 80, -height => 25, -wrap => 'none')->pack(
+-expand => 1);

open(CHILD, "./some_program 2>&1 |") 
                 or die "Can't open: $!";

$mw->fileevent(\*CHILD, 'readable',
                [\&fill_text_widget,$t]);
MainLoop;

sub fill_text_widget {
          my($widget) = @_;

          $_ = <CHILD>;

          $widget->insert('end', $_);
          $widget->yview('end');

}
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