using variable in a switch statement

This is possible. In Tcl, curly braces prevent variable substitution, and you have a set of curly braces surrounding the entire body of your switch statement, so "/$TEST_VAR/*" won't have its variable substituted. You should be able to fix this just by removing those outer braces around your switch body:

 
   switch -glob [string tolower [HTTP::path]]
      "/$TEST_VAR/*" {
         log local0. "Matched pool 2 paths for [HTTP::uri]"
         pool pool2
      }
      default {
         log local0. "Hit default for [HTTP::uri]"
         pool pool_default
      }