[CDBI] Can't override db_Main

I am trying to override db_Main as suggested in the wiki for use under
mod_perl, but somehow  Ima::DBI keeps overriding my override :-)

I have boiled it down to the following example code, and I only have
this problem if my CDBI subclass is in its own file, so here's that
file:

----
  package My::Test::DBI;
  use warnings;
  use strict;
  use base 'Class::DBI';
  __PACKAGE__->connection('DBI:mysql:database=test','test','');
  sub db_Main{die 'made it!'}
  1;
----

And then the calling script:

----
  use warnings;
  use strict;
  use My::Test::DBI;
  use Data::Dumper;
  print $Class::DBI::VERSION . "\n";
  print(Dumper(My::Test::DBI->db_Main));
----

Here's what happens:

----
  Subroutine My::Test::DBI::db_Main redefined at
/usr/local/share/perl/5.8.4/Ima/\
  DBI.pm line 282.
  0.96
  $VAR1 = bless( {}, 'DBIx::ContextualFetch::db' );
----

I expected the last line to say "made it!" as in my custom db_Main but
it's clearly defaulting to a db_Main installed by Ima::DBI. How do I
avoid this?

What's odd is that my custom db_Main stays intact if I define my CDBI
subclass in the same file as the test script like so:

----
  use warnings;
  use strict;

  use Data::Dumper;

  print(Dumper(My::Test::DBI->db_Main));

  package My::Test::DBI;
  use warnings;
  use strict;
  use base 'Class::DBI';
  __PACKAGE__->connection('DBI:mysql:database=test','test','');
  sub db_Main{die 'made it!'}
----

And here's what happens:

----
  made it! at bin/dev/cdbi_dbmain.pl line 17.
----

Not sure what is going on but seems to have something to do with how
the modules are being loaded.