The Google Toolbar is an ActiveX control and is IE specific. You could concievably write one in Perl but it wouldn't be the best way to do it. As someone else mentioned, you can add components to Mozilla with some JavaScript and XUL. There is also creating bookmarks that are JavasSript links. The Google Buttons work like this. If you want to write something that extends the browser and runs on client, Perl is probably not the right choice.