I know you mentioned that you are using perl/cgi, but if you are running this under mod_perl, then you can use the mod_perl hooks to register a cleanup handler to do your post request processing. This way Apache will close the connection for you , and then afterwards execute your cleanup handler for you.

If you are using the mod_perl Apache::Registry module to run your cgi/perl script, then you could add something like this to your code:

  my $r = Apache->request();
  my $sub = sub {
    # Do your post request processing here
  };

  $r->register_cleanup($sub);
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Of course, your code will be dependant on mod_perl at this point and it won't run as a plain old CGI anymore, but I thought it was worth noting as an option.