what is the difference to an online compiler like www.tutorialspoint.com/execute_perl_online.php

That site, like many other similar ones, has to communicate with the server to execute Perl there, while WebPerl runs entirely in the browser, as LanX said. The browser is by default a sandboxed environment, while to run untrusted code on the server side safely takes a fair amount of work to get the security right.

The "WebPerl Code Demo Editor" from the root node is even designed to be run in an <iframe sandbox="allow-scripts">, meaning that if you embed the iframes in your site, if supported by the browser, it will apply cross-origin restrictions and the Perl code won't be able to access any resources from your site's origin and be restricted entirely to the frame. Notice how here, you may not even be able to click on the "Powered by WebPerl" links.

Also, the kind of stuff I showed in the root node is a cool way to show off WebPerl, but IMHO an even more useful application is that you can replace JavaScript with Perl in HTML pages. For example, the WebPerl Regex Tester is written in Perl instead of JavaScript.