If the login is done through a netscape-style userid/password popup then you can pass your userid & password by setting your credentials using the HTTP::Request authorization_basic method. The lwpcook documentation that comes with LWP has a good example.

If the login is done with a userid/password form and cookies, you can use the HTTP::Cookies to set up a cookiejar and then simulate your login. You set up a cookie-jar for an LWP::UserAgent as follows:

use HTTP::Cookies;
my $agent = new LWP::UserAgent;
my $co=new HTTP::Cookies(file=>'./stored.cookies',autosave=>1);
$agent->cookie_jar($co);
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There are other ways that sites can use to do user/password login authentication, but they are almost never user. Those two above cover about %99 of all sites that do some sort of login.

Just an update: HTTPSniffer.pl has moved here.

If your page has a HTML form with a submit button and uses the POST method you can solve your problem very simply. Here is an example from the lwpcook manpage:

use HTTP::Request::Common qw(POST);
use LWP::UserAgent;
$ua = new LWP::UserAgent;

my $req = POST 'http://www.perl.com/cgi-bin/BugGlimpse',
              [ search => 'www', errors => 0 ];

print $ua->request($req)->as_string;
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All you have to do is set up a POST request message with all the fields in your HTML form (check boxes, text fields, and so on), with the correct URL. I hope this helps.
marcos

If your page has (except username/password) buttons and checkboxes, then it's a cgi script. Just see which is the URL of the page called (or even better the form "action" as it might redirect you). Then call that URL (or the action script) with the params you want on the Location bar, thus simulating a form GET. Even if it posts the form with a PUT, it should accept the parameters on the url if it's written with CGI.pm (I don't know about others). If that doesn't work, then I' m afraid I cannot help.