You will have to add a handler to your Apache configuration.

The Apache docs have the following example:

Modifying static content using a CGI script

The following directives will cause requests for files with the html extension to trigger the launch of the footer.pl CGI script.

Action add-footer /cgi-bin/footer.pl
AddHandler add-footer .html
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Then the CGI script is responsible for sending the originally requested document (pointed to by the PATH_TRANSLATED environment variable) and making whatever modifications or additions are desired.

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If you are running mod_perl then you can do much more. The mod_perl docs have the following:

PerlResponseHandler

The handler (response) phase is used for generating the response. This is arguably the most important phase and most of the existing Apache modules do most of their work at this phase.

This is the only phase that requires two directives under mod_perl. For example:

  <Location /perl>
     SetHandler perl-script
     PerlResponseHandler MyApache2::WorldDomination
  </Location>
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SetHandler set to perl-script or modperl tells Apache that mod_perl is going to handle the response generation. PerlResponseHandler tells mod_perl which callback is going to do the job.