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} );

##</code><code>##

$headers->{Content}->{'foo'} = ['bar', 'baz']

##</code><code>##

"field3": "HASH(0x2308c10)"

##</code><code>##

$args = {data => $args};
my @newstring;
foreach my $key (keys %{$args}) {
    foreach my $value (keys %{$args->{$key}}) {
        push @newstring, $key.'['.$value.']='.$args->{$key}->{$value};
    }
}
$args = join('&',@newstring);
my $response = $ua->post( $url, Content => $args );