Irule with if-elseif to change to Switch-glob

Catch can only ever return 0 or 1 based on whether the command it's executing returns an error or not, so I don't think there is any advantage to using switch. But if you wanted to, you could do this:

set pool_name p_test_$TEST
switch [catch { pool $pool_name } ] {
   0 {
       No error, so the pool was already assigned and you don't have to do anything more
   }
   1 {
       Error, so do something else
      Execute some commands
   }
}

Note that catch will run the command in the braces. So you don't have to call the pool command again. You could simplify your original rule like this:

set pool_name p_test_$TEST
if { [catch { pool $pool_name } ] } {
    Error assigning pool $pool_name, so do something else
}

Aaron