in reply to
Access value of a variable after exit 0 in fork
The simplest would be to return a small numeric value via the exit code of the sub-process, read that in the parent process, and translate that in you main code to set $trigger to
The problem you currently have is that $trigger in the child process is in a different memory space, and modifying that in the child has no effect on the parent's instance of $trigger.
"By three methods we may learn wisdom: First, by reflection, which is noblest; Second, by imitation, which is easiest; and third by experience, which is the bitterest."