When I click a link in a Perform activity, nothing happens.

This may be because the link contains special characters. Totara checks to see if any link added is in a valid format. If the link is seen as invalid by Totara, it will remove it.

If this is the case, when editing the link, you will see that the URL box is empty. 

The solution 

First, double-check that the link is valid.  If so, you can use a service such as https://tinyurl.com/ to get a shorter, Perform-friendly URL to paste into Totara.