'field2' => [{'subfield1' => 'x', 'subfield2' => 'y'}, possibly more in array],

See HTTP::Request::Common, LWP doesn't accept/process hash refs , it doesn't have a concept of "subfields", like a regular html form its just key/value pairs. Specifically LWP* wants a flat list like this

[ 
'field1' => '1',
'field2{subfield1]' => 'x', 
'field2[subfield2]' => 'y',
'field3{subfield3]' => 'int',
'field3{subfield4}' => 'string',
 ]
[download]

What you're looking for is like the opposite of CGI::Struct , it is PHP::HTTPBuildQuery / http_build_query

update: An example of a round trip with PHP::ParseStr/php_parse_str :)

update: and now with "LWP"

#!/usr/bin/perl --
use strict;
use warnings;
use PHP::HTTPBuildQuery qw/ http_build_query /;
use PHP::ParseStr qw( php_parse_str );
use Data::Dump qw/ dd /;
use WWW::Mechanize;
my $con = {
    'field1' => '1',
    'field2' => [ { 'subfield1' => 'x', 'subfield2' => 'y' }, ],
    'field3' => { subfield3 => 'int', 'subfield4' => 'string' },
};
my $hbq = http_build_query($con);
my $pps = php_parse_str($hbq);
dd( $con, $hbq, $pps );
my $ua = WWW::Mechanize->new( autocheck => 0 );
$ua->timeout(0.1);
$ua->add_handler("request_send",  sub { shift->dump; return });
#~ $ua->add_handler("response_done", sub { shift->dump; return });
#~ $ua->show_progress(1);
$ua->post( 'http://localhost/', Content => $hbq, );
$ua->post( 'http://localhost/', Content => [ 
"field1" => 1,
"field2[0][subfield1]" => "x",
"field2[0][subfield2]" => "y",
"field3[subfield3]" => "int",
"field3[subfield4]" => "string",
], );
__END__
(
  {
    field1 => 1,
    field2 => [{ subfield1 => "x", subfield2 => "y" }],
    field3 => { subfield3 => "int", subfield4 => "string" },
  },
  "field1=1&field2%5B0%5D%5Bsubfield2%5D=y&field2%5B0%5D%5Bsubfield1%5
+D=x&field3%5Bsubfield3%5D=int&field3%5Bsubfield4%5D=string",
  {
    field1 => 1,
    field2 => [{ subfield1 => "x", subfield2 => "y" }],
    field3 => { subfield3 => "int", subfield4 => "string" },
  },
)
POST http://localhost/
Accept-Encoding: gzip
User-Agent: WWW-Mechanize/1.73
Content-Length: 125
Content-Type: application/x-www-form-urlencoded

field1=1&field2%5B0%5D%5Bsubfield2%5D=y&field2%5B0%5D%5Bsubfield1%5D=x
+&field3%5Bsubfield3%5D=int&field3%5Bsubfield4%5D=string
POST http://localhost/
Accept-Encoding: gzip
User-Agent: WWW-Mechanize/1.73
Content-Length: 125
Content-Type: application/x-www-form-urlencoded

field1=1&field2%5B0%5D%5Bsubfield1%5D=x&field2%5B0%5D%5Bsubfield2%5D=y
+&field3%5Bsubfield3%5D=int&field3%5Bsubfield4%5D=string
[download]

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