It's described in the SOAP::Lite guide.

In the example given:

    -> on_fault(sub { my($soap, $res) = @_; 
         die ref $res ? $res->faultstring : $soap->transport->status, 
+"\n";
       })
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It'll die with the faultstring if there's an SOAP fault, or with the transport status if there wasn't a result object. (and in the case of a failed connection, it'll be a transport error, with no result object.)