open(SUB, "sub.pl |") or die $!

The "|" at the end tells Perl to do a fork()/exec() and hook up STDOUT from sub.pl to your SUB filehandle. Then you can do:

   while (my $line = <SUB>) {
     # do something with $line...

     # then print it out:
     print $line;
   }
[download]

Give it a try and post again if you run into trouble.

-sam