That is the default behavior for a form. The following is the source code for the PerlMonks Search. If you enter some text on the input field and then press the enter key, the form is submited.

          <form method="get" action="?"
              enctype="application/x-www-form-urlencoded" >
            &nbsp;
            <input type="text" name="node"
                id="search_text"
                size="20" maxlength="300" />
            <input class="titlebar" type="submit"
                value="Search" />
          </form>
[download]