Socket is irrelevant. Try: 
perl -e'while(1) { print "Datagram"; sleep 1; }'

[download]
check man stdio for your system. Mine (GNU/Linux) says:
Output streams that refer to terminal devices are always line buffered by default; pending output to such streams is written automatically whenever an input stream that refers to a terminal device is read. In cases where a large amount of computation is done after printing part of a line on an output terminal, it is necessary to fflush(3) the standard output before going off and computing so that the output will appear.

In reply to Re^3: UDP Server doesn't receive before newline by zwon
in thread UDP Server doesn't receive before newline by nitin1704

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