Past Simple can be used instead of Past Perfect in sentences with
after, as soon as, before etc. That makes sense as it is clear which action follows which.
- After it stopped raining, we decided to take a walk in the park.
- As soon as you called, I went straight to bed.
However, Past Perfect will be preferable if we need to emphasise the
distance between separate events.
- As soon as he had graduated, he went travelling for a year.