my $count = Core::Models->resultset('Shipment')->search(
  undef,
  { prefetch => 'rma' }
)->count();

##</code><code>##

package Models::Result::Shipment;

use strict;

use base 'DBIx::Class::Core';

__PACKAGE__->table("shipment");

__PACKAGE__->add_columns(
  "id",
  { 
    accesor => 'id', data_type => "integer", 
    extra => { unsigned => 1 }, is_nullable => 0, 
    is_auto_increment => 1 
  },
  "ref",
  { 
    accesor => 'reference', data_type => "varchar", 
    is_nullable => 1, size => 255 
  },
  "api_merchant_id",
  { 
    data_type => "integer", extra => { unsigned => 1 }, 
    is_nullable => 0, default_value => 1 
  },
  # ... lot of other fields bellow
);

__PACKAGE__->set_primary_key("id");

__PACKAGE__->has_many(
  "rma",
  "Models::Result::RMA",
  {
    "foreign.shipment_reference" => "self.ref",
    "foreign.merchant_id"    => "self.merchant_id",
  },
  { cascade_delete => 0 }
);

1;

package Models::Result::RMA;

use strict;

use base 'DBIx::Class::Core';

__PACKAGE__->table('api_rma');

__PACKAGE__->add_columns(
    'id'                  => { data_type => 'integer', is_nullable => 0, extra => { unsigned => 1 } },
    'shipment_reference'  => { data_type => 'varchar', is_nullable => 0, size => 255 },
    'merchant_id'     => { data_type => "integer", extra => { unsigned => 1 }, is_nullable => 0 },
    # ... lot of other fields bellow
);

__PACKAGE__->set_primary_key('id');

1;


##</code><code>##

$count = Core::Models->resultset('Shipment')->search(
  undef,
  { 
    prefetch => 'rma',
    select => { count => 'DISTINCT(me.id)' },
    as => [ 'total_count' ],
  }
)->get_column('total_count')->first();