To answer some of your questions in kind of random order:

When declaring a relationship, if you know you want it to use an inner join, you can specify that in the relationship declaration:

__PACKAGE__->has_many( 'foos', 'MySchema::Foo', 'bar_id', {
    join_type => 'INNER'
} );
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To find out exactly what queries are being issued as a result of your code, set the environment variable DBIC_TRACE to a true value, and the generated queries will get dumped to stderr.

As for whether it's efficient or not, that depends a lot on the application, the data, and the types of queries you are doing, as well as how you define efficient. I'm sure that I could get queries that ran a little bit faster by building them by hand, but the time that it takes to do so wouldn't offset the time that DBIx::Class saves me, so while the queries themselves might be slightly less efficient, I am much more efficient with it than without.