Variable Scope in iRules

You can use static to set it when the rule initializes: https://devcentral.f5.com/wiki/iRules.static.ashx

when RULE_INIT {
  set static::TEST 0
}

when HTTP_REQUEST {
       if { $static::TEST } {
              do something
       }
}

When HTTP_RESPONSE {
       if { $static::TEST } {
              do something
       }
}