buffering of perl linux is different from perl windows

Using SMTP on my Windows test server outputs line by line the sending of an email using:
use IO::Handle;
select STDOUT;
STDOUT->autoflush();
$|=1;

The same code on the Linux server buffers the output and causes regularaly the error:
504 Gateway time-out

I am still investigating why but looking for some help too.
Thanks,
TomS