Some notes:

When I started my mod_perl app. framework, I'v challenged BEGIN blocks too. Unfortunately, I do not remember, why I left this idea. Well, my part of httpd.conf looks like

#apache 1.3
PerlSetEnv FRAMEWORK_HOME /usr/local/framework
<perl>
  use lib "$ENV{FRAMEWORK_HOME}/perl";
  use MyFramework ();
  MyFramework::new();
# this new() writes the created instance into
# $MyFramework::inst, which is our :)
</perl>
[download]

And what about global vars. I use them with this rule: Write it when apache starts (all actions are caused under MyFramework::new()) and never later. It gives the chance to share occupied memory between childs. I use $ENV{runtime} for volatile (request related) data.

I'v found this question when I supersearched caching in apache, so, when I find something important about it, I'll link it here.