Is it possible to use F5 as proxy/reverse proxy server?

I am not entirely sure what you are trying to do. But you seem to be using TCL lists and I think you are trying to perform double variable expansion. Perhaps this example will help:

 
 % set foo { 1 2 3 4 } 
  1 2 3 4 
 % puts [lindex $foo 3] 
 4 
 % set bar foo 
 foo 
 % puts [lindex [subst $$bar] 3] 
 4 
 

You also might consider using the foreach command to iterate over all the elements of the list instead of the while command.