The first thing to try is to analyse the login procedure using a tool like Mozilla (or Firefox) with the 'Live HTTP Headers' plugin loaded. This will help you to track the exchange between a browser and the site. You can then modify your script to work in the same way.
You should also look at WWW::Mechanize as it is specially designed for this sort of thing.
Other points to consider:
- Does the site require cookies? You will need to add a cookie jar if it does.
- Does the site object to non-browser agents? You will need to change the agent name if it does.
- Does the POSTed data cause the browser to redirect? This is something that you will need to prompt LWP to do.
Can you also post the code that you tried so that we can see what is going on!