How can I purge a package using the apt_get package_module? policy => purge doesn't work.

present and absent are the package states, purge is an option for package manager. You should be able to use the options attribute since the =apt_get= package module supports it.

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  bundle agent main
  {
    packages:

        # Purging a package using the --purge option
        "screen"
          policy => "absent",
          package_module => apt_get,
          options => { "--remove" };
  }   

Note that you can use any Configuration Item as well:

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  bundle agent main
  {
        # Purging a package using the Configuration Item option
        "screen"
          policy => "absent",
          package_module => apt_get,
          options => { "-o", "APT::Get::Purge=true" };
  }