It's really common with this seat, which version of the Pearl do you have? Is it the one with the extra insert for babies? If not, you can look on the Maxi Cosi website and get the "Pearl Comfort Insert" which can help reduce the angle for younger babies.
Have you reclined the seat? Reclining it should help with head flop. However, if you have very slanted back seats and/or low down isofix points, this will cause the angle to be much steeper when the seat is rear facing (which it should be at 9mo 
) so the recline is sometimes not sufficient - there is nothing you can do about this, unfortunately. The only solution is a different seat which allows more of a recline, perhaps a seatbelt fitted one. But IME this is one of the less-reclined isofix seats.
Also, they should not be pulled down in these seats as in the baby seats - they are designed more to sit upright in. Check that your headrest is in the right place. It needs to be just above the child's shoulders as if it's too low it will push them out of place.
And then lastly babies of this age tend to be into sitting upright and they will try and sit bolt upright in the car seat, even though there's actually a bit of a recline on it, making a gap between their back and the seat itself. This can cause a situation where you're strapping them in and their shoulders almost make a sort of "tent" with the straps (it's less dramatic than this mental image!) That can cause them to be further forward than they should be when they fall asleep. To solve this, get them settled into the seat with straps fastened but only loosely over them. At this point they will usually be either bolt upright or bent over forwards trying to grab the buckle that you just did something interesting with and/or their own toes, which are also very interesting to them at this age. So do something above their head to interest them instead. You might dangle a toy here, or move your fingers around interestingly, or have something make a sound. That should get them to look up and, naturally, lean back into the slope of the seat. Use your other hand to tighten the straps at this point, and they should come nicely into the right place where their back is fully in contact with the seat so the straps pin their shoulders back here. If baby is awake and unlikely to sleep, then you can also un-recline the seat so that they can sit more upright, but try to get their back fully in contact with the seat.