Yahoo::Search::Request is a container object for a Yahoo! Search request. (This package is included in, and automatically loaded by, the Yahoo::Search package.)
You never need to use this package directly -- it is loaded automatically by Yahoo::Search.
In practice, this class is generally not dealt with explicitly, but rather implicitly via functions in Yahoo::Search such as Query and Links, which build and use a Request object under the hood.
You also have access to the Request object via Request() method of resulting Response and Result objects.
To be clear, Request objects are created by the Request() method of a Search Engine object (Yahoo::Search).