When Solomon Perel died this past February in Jerusalem, he did not die alone. With him went the alter ego he’d created more than half a century before, when as a Jewish teenager he survived the Holocaust by hiding in plain sight as “Jupp,” a member of the Hitler Youth. In his many postwar decades living as a citizen of Israel, Perel would give interviews in which he swore, up and down, that Jupp was a fully formed personality that took over his body—and that never fully left him. Perel’s story was immortalized in one of the great Jewish films,...