As ikegami pointed out , you should be more specific , so because you are not specific I will make what I want of your question and consider that you want to write some Perl script that will use WWW::Selenium to control Selenium which will in turn control a browser by injecting js in it at runtime. So , quoting the WWW::Selenium documentation :

1)$sel->get_eval($script)

Gets the result of evaluating the specified JavaScript snippet.
[download]

2)$sel->run_script($script)

Creates a new "script" tag in the body of the current test window, and
+ adds the specified text into the body of the command. Scripts run in
+this way can often be debugged more easily than scripts executed usin
+gSelenium's "getEval" command. Beware that JS exceptions thrown in th
+ese scripttags aren't managed by Selenium, so you should probably wra
+p your scriptin try/catch blocks if there is any chance that the scri
+pt will throwan exception.
[download]

You can download Selenium here , you want to download Selenium RC. Good luck !