perlop mentions that unary + has no effect, even on strings. SOAP::Lite::import doesn't seem to care about plus or minus symbols in that initial import list item.

Near as I can tell, the '+' is there because the module author thought it was pretty.

There may be more to it than that, though. I have worked at places where they insisted on prefixing hash keys with a '+' symbol. I never figured out why, and it was a habit I stayed away from. There's enough of a noise problem with my code as it is.