package test_setup;
sub setup {
# setup your db or whatever here
}
1;

##</code><code>##

require t::test_setup;
test_setup::setup();

##</code><code>##

package test_setup;

sub setup {
# ...
}

BEGIN { # must be after the sub definition
  setup();
  $ENV{USE_TEST_DB} = 1;
}

1;

##</code><code>##

if ( $ENV{USE_TEST_DB} ) {
  __PACKAGE__->config( 'Plugin::ConfigLoader' => { file => 'myapp_test.yml'} );
}