Squirrel

In Rise of Nations Scripts, there is a language feature called "trigger"

trigger(/* conditions under which trigger wil fire*/)
{
    //Trigger body, just like function body
}

Each trigger is executed at least once (ofc if it isn't disabled by
default). After trigger has fired, it becomes disabled and will not
fire again, unless enable_trigger(MyTriggerName) function is used in the
body of the trigger.

In Squirrel, where do I get similar functionality? If there is none, then
what is solution for this situation?
 

Can you explain how they work a bit more? What triggers them to begin with?

Okay, here is how they look like and what they do.

Syntax
Trigger's syntax is very similar to that of funtion.
trigger trigger_name([conditions_under_wich_trigger_will_fire)
{
     //List of actions to perform every time trigger fires, just like any function's body

    /*Optional: trigger control statement, either enable_trigger() or disable_trigger(), becasue triggers  only fire once. If we want trigger to fire again, we should place enable_trigger() in the end */
}

Let us create a trigger wich will check victory/defeat conditions.
trigger VictoryCheck(GetPlayerUnitsCount <= 0)   /*under this condition trigger will fire*/
{
    Defeat();
}

Here is another trigger which we want to fire every game tick.
trigger EternalTrigger() /*since we want trigger to fire every tick, no condition required*/
{
    print("Eternal Trigger ticked");
    enable_trigger(); //It enables trigger to fire once again
}

Now, we can disable EternalTrigger from any place in our script. It can be done from both funtions and another triggers:
function LetUsDisableOurTrigger()
{
    disable_trigger("EternalTrigger"); /*Eternal trigger will not tick anymore, unless we enable_trigger() it*/
}
or we can disable it from another trigger
trigger TriggerDisablingAnotherTrigger()
{
    disable_trigger("EternalTrigger");
    /*Since triggers only fire once, this trigger won't fire again,           unless we  allow it to by using enable_trigger() */
}

You have got the idea, haven't you?