I drive a 2009 Subaru Impreza, Sometimes when I start my car a puff of smoke comes out the exhaust, other than that it drives fine?

Keep an eye on the oil level. Check the dipstick every 500 miles just in case your motor is starting to burn oil. If the coolant is getting low the head gaskets may be starting to go.

It may be oil getting past the oil rings and blowing out when the engine starts.

And your question IS...???

Fail

All boxer engines (i.e. horizontally-opposed pistons) do that occasionally.

It only takes a tiny bit of oil to seep past the piston rings while the car is idle and you get that.

My Citroën 2CV did it, so did my Porsche Boxster 986 (far more frequently, but that has six larger cylinders instead of two tiny ones). Classic Beetles also do it as does the original Fiat 500 and the later Fiat 126.

Subaru is famous for it’s flat-4 boxer engines and they’re no exception. As long as it doesn’t occur frequently it’s nothing to worry about. If it is occurring a lot then first check the crankcase venting system isn’t blocked and isn’t running at excessive pressure, and if not then do a compression test (dry, then repeated “wet”) to check for worn piston rings or damaged valve seats. If those tests prove negative then if the problem persists you’ll need to remove at least one of the cylinder heads to check the piston bores for excessive wear.