JavaScript and forms

A common mitigation against automated agents or scripts using curl is to have the page with the HTML form use JavaScript to set values of some input fields, usually one of the hidden ones. Often, there is some JavaScript code that executes on page load or when the submit button is pressed which sets a magic value that the server then can verify before it considers the submission to be valid.

You can usually work around that by reading the JavaScript code and redoing that logic in your script. Using the tricks in Figure out what a browser sends to check exactly what a browser sends is then also a good help.