the error message that I am forwarding in my posts is the status_line of the response object, not that of the user agent object.

You are mistaken. It's LWP::UserAgent that places the error in the status line (after being generated by LWP::Protocol::http). That is how LWP reports errors.

So yes, we were on the same page. Maybe older versions of LWP had different messages, sorry.

Update: It would have to be a very old version. Older than libwww-perl-5.800, released on June 15th, 2004. That's the earliest version currently on CPAN.