When using virtual hosts, any configuration directive outside of a <VirtualHost> container is applied to a virtual host called the main server, which plays a special role. First, it acts as the default host when you're using name-based virtual hosts and a request can't be mapped to any of the configured virtual hosts (for example, if no Host: header is provided). Secondly, many directives specified for the main server are merged with directives provided in <VirtualHost> containers. In other words, virtual hosts inherit properties from the main server. This allows us to specify default behaviors that will apply to all virtual hosts, while still allowing us to override these behaviors for specific virtual hosts.
In the following example, we use the PerlSetupEnv directive to turn off environment population for all virtual hosts, except for the www.example.com virtual host, which needs it for its legacy CGI scripts running under Apache::Registry:
PerlSetupEnv Off Listen 8001 <VirtualHost 127.0.0.1:8001> ServerName www.example.com PerlSetupEnv On </VirtualHost>