Retrieving data from Agilent 34401A Digital Multimeter over the RS232 interface
The problem was in the way I send a command over terminal. After a command the multimeter expects line feed character, in PuTTY this is &lt;ctrl-j&gt;. 

The code below successfully gets response with a value of a voltage. 

syst:rem; 
:meas:volt:dc? 10, 0.003;
&lt;ctrl-j&gt;


I found solution reading through this forum question:
https://www.eevblog.com/forum/testgear/34401a-serial-interface-problems/