Click SOAP::Lite then click the "Source" link near the top to get to Lite.pm. Search for "sub typecast" and then search backward for /^package/ and you'll find the "package SOAP::Deserializer;" line. Thus the "sub typecast" is where the original SOAP::Deserializer::typecast() is defined. And, no, "typecast" is not a special keyword. Search for uses of "typecast" elsewhere in that source code.
I suspect (but haven't verified) that "they" expect you to "replace" the typecast() functionality by subclassing (inheritance) rather than how it is attempted in the root of this thread.