Given a perl script test.pl:

#!/usr/bin/perl
$| = 1;   # immediate flush
for ($i=0;$i<10;$i++) {
  print "*";
  sleep(2);
}
exit;

And a php script test.php:

<?
  header("Content-Type: text/plain"); 
  ob_immediate_flush;   # here or in php.ini
  echo "calling test.pl\n";
  passthru("/path/to/test.pl");
  echo "finished\n";
?>

Eventhough both scripts have buffering disabled, no progressive content loading happens. PHP autoflushes the first echo-line, but passthru-output is not flushed until the test.pl script has completed. 

There are situations where this behavior is no good. I have perl-scripts doing jobs on our server. These perl-scripts are called from a webpage (aka php-script). However, an inbetween firewall shuts down communication after 5 mins with no data transferred. In order to keep the connection alive, I rewrote the perl-scripts such as to send a progress character to the browser every 30 seconds. Not being autoflushed, they don't keep alive anything :-)