Documentation for prefix
-- assembled from the following types:
multi sub prefix:<-->( is rw) is assoc<non>
Decrements its argument by one and returns the updated value.
my = 3;say --; # OUTPUT: «2␤»say ; # OUTPUT: «2␤»
It works by calling the pred method (for predecessor) on its argument, which gives custom types the freedom to implement their own decrement semantics.