What is the best way to trigger a customized pgm immediately from sales order P4210?

To avoid customization into sales order entry P4210. We wanted to create a batch pgm to be called from sales order entry.

I will like to know the best way not to customized P4210 to call this program . Any possibility to leverage on other area e.g subsystem or whichever technology to supplement this area?

Appreciate advise from the group

my version : E1 9.1 . Tool set is the latest one.