$soap_req =~ s/</&lt;/g;

 ...

 my $som = $soap->call(SOAP::Data->name('apple:getanswer')
                                 ->attr({"xmlns:apple" => $ns_apple
                                         })
                                 =>SOAP::Data->name('soap')
                                             ->type('xsd:string'=> $soap_req)
                       );