This isn't really an AMS question; look in your 1364 Verilog reference.
The "<=" construct is a non-blocking assignment, but usually you see something like

Code:

A <= #10 1;
B <= #5 0;
 

With the non-blocking assignment, B gets the value 0 at time 5, rather than time 15 if a regular assignment had been used.  I've never seen a delay specified as 'B as in your example, rather than as a number.  The LRM allows a number or an identifier, not an expression like 'B.