my $type = 'post';
my $url = redacted;
my $headers = {
  'Content' => {
    'field1' => '1',
    'field2' => [{'subfield1' => 'x', 'subfield2' => 'y'}, possibly more in array],
    'field3' => {subfield3 => int, 'subfield4' => 'string'},
  },
}
my $response = $ua->$type( $url, %{$headers} );