#!/usr/bin/perl -w
use strict;

use Tk;

my $mw = MainWindow->new;

my $main_frame = $mw->Frame()->pack(-side=>'top',-fill=>'x');
my $go_button = $main_frame->Button(-text=>'Go Button',
              -command => \&do_command)->pack();

MainLoop;

sub do_command
{
   # create and display text widget
   my $tw = $main_frame->Text()->pack();

   open (COPY_PROJ, '-|', 'perl testing123.pl') or die "unable to start $!";

   my $first_line = "Copying project...please wait";
   $tw->insert( 'end', $first_line );
   
   my $copy_proj_line;
   
   while (defined ($copy_proj_line =<COPY_PROJ>) ) 
   {
       $tw->insert( 'end', $copy_proj_line );
       $tw->update(); ### this is the key to see values as they come
   }   
}

__END__

file: testing123.pl:

#!/usr/bin/perl -w
use strict;
$|=1;  #no buffering on stdout

for (1..10)
{ 
   print "line number $_\n";
   sleep 1 if (rand() > 0.5);
}