Create a ruby Proc from a string

I want to define the block as a string, then create the lambda. The following example does not work. Is something like this possible?

code_string = "|x|x*2"

l = lambda {eval(code_string)}

l.call(3) => 6

This works

eval  "lambda { " + code_string + " }"

I just don't know why this one does and the other does not.

eval "lambda {#{code_string}}"