From the Safe perldoc

reval (STRING)
    This evaluates STRING as perl code inside the compartment.

The code can only see the compartment's namespace (as returned by the 
root method). The compartment's root package appears to be the main:: 
package to the code inside the compartment.
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so the code in the compartment can't see any other namespaces so it doesn't know that the Person namespace exists. Try

$string = <<'EOM';
  use Person;
  $data = {
          'person' => bless( [
                       42
                     ], 'Person' )
        };
EOM
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--

flounder