Global variable?

Elastic Stack Logstash
2

I’ve tried this code but it doesn’t work

date {
        match => ["period1","dd/MM/YYYY"]
        target=> "period1"
    }
    date {
        match => ["period2","dd/MM/YYYY"]
        target=> "period2"
   }
   ruby {
        code => "event.set('period', (event.get('period2') - event.get('period1'))/3600/24)"

   }
   ruby{
               code => "$my_var " = "%{peroid}" 
       }

I want to use the value of $my_var as a condition for an other event (next line)