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First number: 
<input type="text" id="num1"  />
...
<input type="text" id="answer" />
Add them:
<input type="button" value="Add" onclick="addthem();" />

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

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

use HTML::Template;
use CGI;
...
$template->param( answer => $num1 + $num2 )
print "Content-type: text/html\n\n";
print $template->output();