Not refrigerating leftover canned food is not worth the risk of your cat getting sick.
Always refrigerate the unused portion of canned food.
This is how I do it....
AM meal / my 2 cats share 1/2 can divided into 2 bowls.
I transfer the remaining half to a small glass storage container with a tight plastic lid which I immediately refrigerate.
PM meal / they eat the remaining half of the canned food.
I remove the refrigerated food, divide that into two microwaveable dishes.
I microwave those for 10 seconds to take the chill off.
This works really well for us.