From language/operators

See primary documentation in context for prefix //

multi sub prefix:<//>(Any --> Bool:D)

Boolean context operator.

Available as of 6.e language version (early implementation exists in Rakudo compiler 2022.12+).

Coerces the argument to Bool by calling the defined method on it.

From language/operators

See primary documentation in context for infix //

The defined-or operator or infix // returns the first defined operand, or else the last operand. Short-circuits.

say Any // 0 // 42;         # OUTPUT: «0␤»