Server-side includes (SSI) are tags embedded directly into an HTML file that perform special functions. They are most commonly used for running CGI scripts and displaying the result; most web page counters are performed using SSI.
The Apache::Include module lets you include entire Apache::Registry scripts:
You could have used standard SSI to include a CGI script for the same purpose, but this way is faster. To use mod_include with mod_perl, you need to configure mod_perl to do so at compile time.