Physical layer network coding (PNC) has been proposed for next generation networks. In this contribution, we investigate PNC schemes with embedded perfect secrecy by exploiting structured interference in relay networks with two users and a single relay. In a practical scenario where both users employ finite and uniform signal input distributions we propose upper bounds (UBs) on the achievable perfect secrecy rates and make these explicit when PAM modems are used. We then describe two simple, explicit encoders that can achieve perfect secrecy rates close to these UBs with respect to an untrustworthy relay in the single antenna and single relay setting. Lastly, we generalize our system to a MIMO relay channel where the relay has more antennas than the users and optimal precoding matrices which maintain a required secrecy constraint are studied. Our results establish that the design of PNC transmission schemes with enhanced throughput and guaranteed data confidentiality is feasible in next generation systems.