No, the + is just the unary + operator applied to the bareword which is just the bareword itself; it doesn't make it part of the argument.

$ perl -le 'BEGIN{package Foo; sub import { print join( "\n", @_ ) }; 
+$INC{"Foo.pm"}=1; } use Foo +bar => "baz", -quux => "jinkies";'
Foo
bar
baz
-quux
jinkies
[download]

I think SOAP::Lite uses the + as a visual cue to distinguish it from options which are prefixed by a - to indicate they're disabled (and the fat comma special cases - so that -foo => "blah" does give "-foo", "blah" even though "-" isn't a \w character).