You have just seen how to use a configuration module for configuration centralization and an easy access to the information stored in this module. However, there is somewhat of a chicken-and-egg problem--how to let your other modules know the name of this file? Hardcoding the name is brittle--if you have only a single project it should be fine, but if you have more projects which use different configurations and you will want to reuse their code you will have to find all instances of the hardcoded name and replace it.

Another solution could be to have the same name for a configuration module, like My::Config but putting a different copy of it into different locations. But this won't work under mod_perl because of the namespace collision. You cannot load different modules which uses the same name, only the first one will be loaded.

Luckily, there is another solution which allows us to stay flexible. PerlSetVar comes to rescue. Just like with environment variables, you can set server's global Perl variables which can be retrieved from any module and script. Those statements are placed into the httpd.conf file. For example

  PerlSetVar FooBaseDir       /home/httpd/foo
  PerlSetVar FooConfigModule  Foo::Config

Now I require() the file where the above configuration will be used.

  PerlRequire /home/httpd/perl/startup.pl

In the startup.pl I might have the following code:

    # retrieve the configuration module path
  use Apache:
  my $s             = Apache->server;
  my $base_dir      = $s->dir_config('FooBaseDir')      || '';
  my $config_module = $s->dir_config('FooConfigModule') || '';
  die "FooBaseDir and FooConfigModule aren't set in httpd.conf" 
    unless $base_dir and $config_module;

    # build the real path to the config module
  my $path = "$base_dir/$config_module";
  $path =~ s|::|/|;
  $path .= ".pm";
    # I have something like "/home/httpd/foo/Foo/Config.pm"

    # now I can pull in the configuration module
  require $path;

Now I know the module name and it's loaded, so for example if I need to use some variables stored in this module to open a database connection, I will do:

  Apache::DBI->connect_on_init
  ("DBI:mysql:${$config_module.'::DB_NAME'}::${$config_module.'::SERVER'}",
   ${$config_module.'::USER'},
   ${$config_module.'::USER_PASSWD'},
   {
    PrintError => 1, # warn() on errors
    RaiseError => 0, # don't die on error
    AutoCommit => 1, # commit executes immediately
   }
  );

Where variable like:

  ${$config_module.'::USER'}

In my example are really:

  $Foo::Config::USER

If you want to access these variable from within your code at the run time, instead accessing to the server object $c, use the request object $r:

  my $r = shift;
  my $base_dir      = $r->dir_config('FooBaseDir')      || '';
  my $config_module = $r->dir_config('FooConfigModule') || '';