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get '/my_stuff' => sub {
    my $my_stuff = config->{my_stuff} || {};
...
    set my_stuff => {X => 'newX', Y => 'newY'};
    return 'New stuff configured.';
};

$ cat config.yml
my_stuff: { X: 'oldX' }

$ curl http://localhost:3000/my_stuff
X[oldX] Y[not set]
...

$ curl http://localhost:3000/my_stuff
X[newX] Y[newY]