In the following configuration:

<Files ~ "*\.fltr">
    SetHandler perl-script
    PerlSetVar Filter On
    PerlHandler Filter1 Filter2 Filter3
</Files>

each of the handlers Filter1, Filter2, and Filter3 will make a call to $r->filter_input( ), which will return a file handle. For Filter1, the file handle points to the requested file. For Filter2, the file handle contains whatever Filter1 wrote to STDOUT. For Filter3, it contains whatever Filter2 wrote to STDOUT. The output of Filter3 goes directly to the browser.

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