You got confused there, headers come after params

#!/usr/bin/perl --

use strict;
use warnings;

use HTTP::Request::Common qw' POST ';

my %params= ('page' => 28,
         'index' => 36);

print POST('http://example.org', \%params , qw' Content-Type form-data
+ ', )->as_string;
__END__
POST http://example.org
Content-Length: 132
Content-Type: multipart/form-data; boundary=xYzZY

--xYzZY
Content-Disposition: form-data; name="index"

36
--xYzZY
Content-Disposition: form-data; name="page"

28
--xYzZY--
[download]

That still doesn't help the OP with his custom protocol