Use something like this - it works for me (running Apache):

#!/usr/bin/perl

use CGI;

$| = 1;
my $q = new CGI;
print $q->header(-type => 'text/html');

print "<pre>\n";
print "Running date:\n";
system("date");
print "sleeping...\n"; sleep(10);
print "Running ls:\n";
system("ls");
print "</pre>\n";
[download]

Most http servers now will stream output from CGI scripts to clients. That wasn't always the case in the early years.