Probably using of javascript is the best choice here, but we can do something on sever side:

#!/usr/bin/perl

use warnings;
use strict;


$|=1;

use CGI qw/:standard/;

print header;

print "line one<br>\n";
print ' ' x 4096;
sleep(3);
print "line two<br>\n";
print ' ' x 4096;
sleep(3);
print "line three<br>\n";
print ' ' x 4096;
sleep(3);
print "line four\n";
print "<br>\n";
sleep(3);
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