Its something in CGI.pm.

To wit:

#!/usr/bin/perl -w

$|++;

{
    print "Content-Type:text/html\n\n";

    print "Autoflush is currently ".($|?"on":"off").".\n";
    for ( 1..5 ) {

      print "Value is $_.\n";

      sleep 1;
    }
    print "Autoflush is currently ".($|?"on":"off").".\n";
}
[download]

I don't have time to look exactly - but likely caching or something else. Take a look at the raw headers. blahblah