Depends on what you want to do next. Perl. PHP and Python are designed for creating interactive web sites, while languages like Java and Ruby are designed for creating full scale applications. They are very different things and at some point you will want to know both.
Do you already have a web site or know what host you plan to use for sites? Check and see what languages your host supports, and what scripts may already be provided by your hosting service. If those scripts are in Perl, that would probably be the next language to learn. If they are in PHP or Phython, you would want to learn them instead. It is going to vary by the host you use.
It also depends on whether you want to create your applications to run from the client or the server side. Languages like JavaScript and AJAX are designed to be part of the HTML code for your page, download to the user's pc (the client), and run locally on that pc. While others like Java are designed to run on the server and do not download to the user's pc.
If I had to pick a language, I would probably say to start with JavaScript (which is NOT the same as Java in spite of them having similar names.) It is the oldest of the scripting languages, and so it has more resources, free scripts, tutorials, etc. Plus it is used in both AJAX and RUBY, so by learning JavaScript you learn much of AJAX and RUBY also. Plus the "concepts" you will learn from JavaScript are the same as the other languages you listed. Once you know how to create a loop or to compare variables, learning the differences in syntax (wording/format) for the different languages is easy.