While WWW::Mechanize is the sure way to go if you want to venture down that route, let me add some words of caution:

Your automatic stock trader will rely on market data provided presumably electronically. Please, don't be stupid and make sure that you attribute "freshness" to your market data, so that you don't buy and sell upon stale data that is 15 minutes old (or even older).

I know of commercially sold trading systems, that even are completely scriptable, which do not have any concept of freshness of market data, but which have problems with their interfaces, so that, for example, the buying price might become stale while the selling price still updates. In such a situation, your automatic stock trader would issue buying contracts upon the stale data and sell to prices based on fresh data, and the difference would kill you if it were left to run unsupervised.

I is a sad fact, but there is no money-making machine that simply produces money while you don't watch...

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
[download]