Try printing headers using CGI, for example

use CGI qw//;
my $q = CGI->new();
print $q->header(
    -nph => 0,
    'Content-Type' => ' text/event-stream',
    'Cache-Control' => 'no-cache',
    'Connection' => ' keep-alive',
);
print $q->header(
    -nph => 1,
    'Content-Type' => ' text/event-stream',
    'Cache-Control' => 'no-cache',
    'Connection' => ' keep-alive',
);

__END__
Cache-control: no-cache
Connection:  keep-alive
Content-Type:  text/event-stream; charset=ISO-8859-1

HTTP/1.0 200 OK
Server: cmdline
Date: Fri, 04 Sep 2015 23:33:29 GMT
Cache-control: no-cache
Connection:  keep-alive
Content-Type:  text/event-stream; charset=ISO-8859-1
[download]

Do you see the difference? Without nph off, apache is going to fill in the missing headers

With nph on, apache is not touching the headers .... and probably not buffering