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

In reply to Re^3: HTML5, SSE, flush output '$| = 1' not working as expected by Anonymous Monk
in thread HTML5, SSE, flush output '$| = 1' not working as expected by ehaase

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