Without seeing your code, it is very hard to guess what happens there. I guess from your description that the script prints the message Unexpected field value http://www.thesite.com/members/login.asp at (eval 5) line 1 while running. I don't know what might cause this message to be printed, so I guess that it has something to do with how you are passing form values to the $mech object.
perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ; # The
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' # web
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