get '/my_stuff' => sub {
    my $my_stuff = config->{my_stuff} || {};
    my $x = $my_stuff->{X} || 'not set';
    my $y = $my_stuff->{Y} || 'not set';
    return "X[$x] Y[$y]";
};

get '/my_new_stuff' => sub {
    set my_stuff => {X => 'newX', Y => 'newY'};
    return 'New stuff configured.';
};

##</code><code>##

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

##</code><code>##

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

$ curl http://localhost:3000/my_new_stuff
New stuff configured.

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