undefined function curl_init()

When I try to create a test node of a content type A (which includes scribd field) I get this error:

"Fatal error: Call to undefined function curl_init() in .../sites/all/modules/scribdfield/scribdfield.module on line 169"

In line 169 is the call :
$ch = curl_init(); // slightly simpler than drupal's version.

in function :

function scribdfield_api_upload($file) {
  $params['file']     = '@' . $file;
  $params['method']   = 'docs.upload';
  $params['api_key']  = variable_get('scribd_api_key', NULL);

  $ch = curl_init(); // slightly simpler than drupal's version.
  curl_setopt($ch, CURLOPT_URL, 'http://api.scribd.com/api');
  curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
  curl_setopt($ch, CURLOPT_POST, 1);
  curl_setopt($ch, CURLOPT_POSTFIELDS, $params);
  return new SimpleXMLElement(curl_exec($ch));
}

In my debian server I have installed the package php5-curl