From language/operators

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Same as infix ^^, except with looser precedence.

Returns the operand that evaluates to True in Boolean context, if and only if the other operand evaluates to False in Boolean context. If both operands evaluate to False, returns the last argument. If both operands evaluate to True, returns Nil.

When chaining, returns the operand that evaluates to True, if and only if there is one such operand. If more than one operand is true, it short-circuits after evaluating the second and returns Nil. If all operands are false, returns the last one.