Take a peek in Catalyst::Plugin::ConfigLoader for the many places and ways you can influence configuration. Some version of this should do it.

BEGIN {
  $ENV{MYAPP_CONFIG_LOCAL_SUFFIX} = "_test";
}
# Where myapp_test.yml has the override
# setting (plus whatever _local mods you have too), e.g.-
---
Model::BLAH:
  connection_info:
    - my_override_stuff...
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