Thanks for this. It's not what I ended up with, but it was educational. I was puzzled about how to use dir_config at compile-time, but now I've learned about Apache::ServerUtil so that's nice.

The thing that kept me from using this is that, although I could get some particular value into the script, I couldn't get the environment variable down there. If I added

PerlSetVar ORACLE_SID production
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in the Apache configuration with my handler, I could add

if( $ENV{'MOD_PERL'} } ) {
  require 'Apache2::ServerUtil' ;
  my $s = Apache2::ServerUtil->new ;
  $ENV{'ORACLE_SID'} ||= $s->dir_config('ORACLE_SID') ;
}
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before the set_db call and thing would basically work. But I couldn't figure out how to pull ORACLE_SID from the environment.