Say I have a web service http://www.example.com/webservice.pl?q=google which returns text \"google.com\". I need to call this web service (http://www.example.com/webservice.
EDIT:
It has been a decade since I answered this question and we now have support for cross-domain XHR in the form of CORS.
For any modern app consider using fetch to make your requests. If you need support for older browsers you can add a polyfill.
Original answer:
Keep in mind that you cannot make requests across domains. For example, if your page is on yourexample.com and the web service is on myexample.com you cannot make a request to it directly.
If you do need to make a request like this then you will need to set up a proxy on your server. You would make a request to that proxy page, and it will retrieve the data from the web service and return it to your page.