From the HTTP::Response and HTTP::Message (from which HTTP::Response is descended) documentation:

$r->as_string
    Returns a textual representation of the response.
    Mainly useful for debugging purposes. It takes no
    arguments.

$mess->content([$content])
    The content() method sets the content if an argument
    is given.  If no argument is given the content is not
    touched.  In either case the previous content is
    returned.
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Have you tried both and noted what their results are? I suspect content returns the content of the response, while as_string returns a string representation of the response object as a whole, which likely includes things like HTTP status information and headers. Since most HTTP objects are based upon a textual HTTP message of some kind, you will typically find that they usually support an "as_string" method to un-do that parsing, giving you what should ordinarily be the same textual HTTP response that was given in the first place.