Apache::OutputChain was written to explore the possibilities of stacked handlers in mod_perl. It ties STDOUT to an object that catches the output and makes it easy to build a chain of modules that work on the output data stream.

Examples of modules that are built using this idea are Apache::SSIChain, Apache::GzipChain, and Apache::EmbperlChain—the first processes the SSIs in the stream, the second compresses the output on the fly, and the last provides Embperl processing.

The syntax is like this:

<Files *.html>
    SetHandler perl-script
    PerlHandler Apache::OutputChain Apache::SSIChain Apache::PassHtml
</Files>

The modules are listed in reverse order of their execution—here the Apache::PassHtml module simply collects a file's content and sends it to STDOUT, and then it's processed by Apache::SSIChain, which sends its output to STDOUT again. Then it's processed by Apache::OutputChain, which finally sends the result to the browser.

An alternative to this approach is Apache::Filter, which has a more natural forward configuration order and is easier to interface with other modules.

Apache::OutputChain works with Apache::Registry as well. For example:

Alias /foo /home/httpd/perl/foo
<Location /foo>
    SetHandler "perl-script"
    Options +ExecCGI
    PerlHandler Apache::OutputChain Apache::GzipChain Apache::Registry
</Location>

It's really a regular Apache::Registrysetup, except for the added modules in the PerlHandler line.

Available from CPAN. See the module manpage for more information.