I agree with hippo's suggestion that 'Checkout' should be part of the name. Also

1. Your class is doing checkout handling and trolley handling (Single responsibility pattern / Cohesion). Consider having a separate Trolley object for trolley handling.
2. The pattern of iterating over the trolley and doing something to just one item suggests the trolley ought to be a hash (keyed by the product ID). You can add extra keys to aid with sorting if you need that.
3. Consider using Moo or something similar to make the OOP more ergonomic.
4. Exceptions are usually preferable to silent returns because they are harder to ignore (see Try::Tiny).
5. 'unless' is tricky for many people (it doesn't scale up, can lead to double negatives etc).
6. 'and' and '&&' are not the same thing (the former is for combining statements)