Bingo! The _msg field has clues (apparently sufficient!) to decipher what happened to a request, when combined with the knowledge of whether or not the request timed out. The secrets of the HTTP::response object can be revealed with:

my $x = $ua->simple_request($req);
foreach $k (keys(%$x)) {print "$k = $x->{$k}\n";}
[download]

Is there definitive documentation on the message meanings? I cannot locate the source to HTTP::Protocol::http at search.cpan.org. Reverse engineering is going pretty well, but that's not the preferred method, now is it?