while loop optimization

It looks like you have a single two dimensional array called $static::Pools and a second array called $static::NatPortBlock, and you'd like to get the first unused element in each. I'd recommend that you store the last index of each using a table command. It would look something like

when RULE_INIT {
    set static::Pools {{"a" "b" "c" }  {"d" "e"} {"f" "g" "h" "i"}}
}
when HTTP_REQUEST {
  log local0. "Array size is [llength $static::Pools]"
  set last_dim_one_index [table lookup "last_dim_one_index"]
   if { [string length $last_dim_one_index] == 0  ||
        $last_dim_one_index == [llength $static::Pools]
    } then {
      set last_dim_one_index 0
      table set "last_dim_one_index" 0 indef  indef
} else {
      table incr "last_dim_one_index"
}
   set indexkey "column_of_row_$last_dim_one_index"
   set index_two [table lookup $indexkey]
   if { [string length $index_two] == 0  ||
        $index_two == [llength [lindex $static::Pools $last_dim_one_index]]
    } then {
      set index_two 0
      table set $indexkey 0 indef indef
} else {
      table incr $indexkey
}
set myList [lindex $static::Pools $last_dim_one_index]
set myReturnString "Answer is [lindex $myList $index_two] for $last_dim_one_index and $index_two $indexkey"
HTTP::respond 200 content $myReturnString
}
 

I'm not sure what the relationship between the index of $k and $i or $j is in your example, but I think that in general you can see how to use the table command to simulate your stack. If you'd like to do a "pop", you can just use

 table incr $key -1