You should use protocol method of HTTP::Request, which is inherited from HTTP::Message. The resulting code should be something like this:

#!/usr/bin/perl
use LWP::UserAgent;

$ua = LWP::UserAgent->new;
$ua->agent("$0/0.1 " . $ua->agent);
$req = HTTP::Request->new(GET => 'http://www.perlmonks.org/');

# here is what you need
$req->protocol('HTTP/1.0');

$res = $ua->request($req);
if ($res->is_success) {
        print $res->content;
} else {
        print "Error: " . $res->status_line . "\n";
}
[download]

HTH, Valerio