Specifically, this is an HTML <form> having two <input> boxes, each having distinct names, that are rendered side-by-side.   When the user enters data into one or both fields and then presses some kind of “submit” button (or, the form submits itself using JavaScript), the values in one or both fields will be included in the HTTP GET or POST stream that is sent to the server.   You should also be aware that the value will be sent only if it is not-empty and if it has changed.   (Easily the best way to see this is to use the debugging capability of any browser which will let you see the HTTP exchange that takes place.   Create a form, type something, hit the submit-button, and watch.   Or, check out the web-page referred-to below.)

When the data arrives on the host side and is processed by Perl, anything that is actually present in the stream can be retrieved using its name.

Now, having said all of this, this really is more of an HTML question than strictly a Perl one.   Although server-side languages vary, the fundamentals of what they’re dealing with are not, and this question appears to mostly concern those fundamentals.   Therefore, may I suggest that you spend time at http://www.w3schools.com where a very large amount of good tutorial information is presented in the form of online web tutorials.   (They include interactive forms that allow you to “try it,” and to immediately see the result.)