Handling program output in real time

[dvergin]

Given some program that produces output over time, let's call it "blackbox.pl":

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

for my $step (1..4) {
   sleep 3;
   print "Step $step\n";
}

I had expected (and wanted) the following code to capture and print out that output as it occurred:

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

$|++;
open PIPE1, "./blackbox.pl |";
while ( <PIPE1> ) {
   # ...Filter and munge $_
   print $_;
}
close PIPE1;

But what it does (with or without the $|++) is wait until blackbox.pl finishes and then it prints out all the output at once.

What am I doing wrong? How can I capture and print the output of blackbox.pl as it occurs? (Needless to say, the real program will be doing some filtering and munging along the way.)

TIA, David