Solved it. There is an internal method to HTML::Form called push_input. I just had to use it. It should probably be in the docs for HTML::Form, eh?

my @attr = (
             { 'name'  => '__EVENTTARGET',
               'id'    => '__EVENTTARGET',
               'value' => 'ctl00$cpMain$pagerTop',
             },
             { 'name'  => '__EVENTARGUMENT',
               'id'    => '__EVENTARGUMENT',
               'value' => '2',
             },
           );

for (@attr) {
  $mech->form_number(0)->push_input('hidden', $_);
}
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