Step 1: Separate where each listed language actually executes.
Some web languages run on the server before the page is even sent out to a visitor, while others run inside the visitor's own browser after the page has already loaded.
Step 2: Sort the given options into these two groups.
Java, Ruby, and Perl are all commonly used on the server side to generate pages, connect to databases, and handle backend logic, and none of them execute inside a typical browser.
Step 3: Identify the browser-native language among them.
JavaScript is built directly into every modern web browser and is used to make pages interactive, validate forms, and update content without a full reload, which makes it the classic example of a client-side scripting language.
\[ \boxed{\text{JavaScript}} \]