I tried to use the following line in add.html:
<%include <%filename%>%>

In add.cgi, I tried to pass the value of <%filename%>:
&site_html_add_form ({ filename => 'abc.html', Category => $category, %in }, $dynamic)

Although abc.html is there, I always got this error message:
Can't find file: ....../abc.html

Is there any way to do this? Thanks!