Could you share the screenshot of the code where you assign this var_UID to both objects?
Actually, the name "var_UID" is quite confusing, which object's UID it is supposed to be?
If you want to tie prt_laser to obj_laser, you only need to create this instance variable on one of the objects (prt_laser), not both.
Rename prt_laser.var_UID to prt_laser.obj_laser_UID
When you create your particles, do this:
prt_laser.obj_laser_UID = obj_laser.UID
When you want to pick particles tied to your laser, do this:
prt_laser -> Compare instance variable -> obj_laser_UID = obj_laser.UID
Or you can simply add both objects to the same container. They will be spawned together, destroyed together and when you pick laser instance in your events, its corresponding particles instance will be picked automatically.