I agree with AnomalousMonk:

 text examples in the
    POST $url, Header => Value,...
    POST $url, $form_ref, Header => Value,...
format, 

(Update1: deleted my wrong conclusion)

these are just text / informal examples. They are not actual Perl.

So, the perl function invocation is   $ua->request(POST 'http://somewhere/foo', [foo => bar, bar => foo]); in your case  $ua->request(POST 'http://somewhere/foo', \@fields)

In this Monastery there is no magic (maybe only M.A.G.I.C. or just M-A-G-I-C (re: B-I-N-G-O) ), that's why I am here

p.s. Kudos on your meticulous investigation skills ++ (re: headers byte count).