Here's two ways* I would consider (there are pros and cons to each):

1. HTML and Javascript (fast, client-side, but 10%± have JS turned off):

First number: 
<input type="text" id="num1"  />
Second number: 
<input type="text" id="num2"  />
Your answer:
<input type="text" id="answer" />
Add them:
<input type="button" value="Add" onclick="addthem();" />
[download]

or HTML for CGI handling, change last lines to:

Your answer:
<input type="text" id="answer" value="<tmpl_var answer>" />
<input type="submit" value="Add" />
[download]

Javascript:

function addthem() {
   var num1 = document.getElementById("num1");
   var num2 = document.getElementById("num2");
   var answer = num1 + num2;
   document.getElementById("answer").value = answer;
}
[download]

2. CGI and HTML::Template (doesn't require JS, but does require a module and a round robin to the server):

use HTML::Template;
use CGI;
my $query = new CGI;
my $num1 = $query->param('num1');
my $num2 = $query->param('num2');
my $template = HTML::Template -> new(filename => "addnumbers.tmpl");
$template->param( answer => $num1 + $num2 )
print "Content-type: text/html\n\n";
print $template->output();
[download]

*all code is untested