This is a weird way of passing parameters.

As discussed in rcaputo's reply and the link(s) therefrom, it is another (and exactly equivalent) way of writing
    $kernel->post('ua', 'request', 'got_response', GET $url);
except, IMO, less messy (for some definition of 'mess') and distracting.

Another manifestation of this idiom is in the quasi-self-documentating
    return bless $objref => $class;
which one might find in an object constructor and which might be read as 'bless object reference into class'.