I see that DBIx::Class::Schema::Loader::DBI::mysql uses SHOW CREATE TABLE to get table definition, and then looking for

CONSTRAINT ... FOREIGN KEY ... REFERENCES ...
[download]

to get information about foreign keys. I tried to create some tables in mysql and check the output of SHOW CREATE TABLES:

mysql> create table foo (id integer primary key, name varchar(10));
Query OK, 0 rows affected (0.01 sec)

mysql> create table boo (id integer primary key, foo_id integer refere
+nces foo(id));
Query OK, 0 rows affected (0.00 sec)

mysql> create table bar (id integer primary key, foo_id integer, const
+raint foo_id_fk foreign key (foo_id) references foo(id));
Query OK, 0 rows affected (0.00 sec)

mysql> show create table boo;
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+----------+
| Table | Create Table                                                
+                                                                     
+          |
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+----------+
| boo   | CREATE TABLE `boo` (
  `id` int(11) NOT NULL,
  `foo_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+----------+
1 row in set (0.00 sec)

mysql> show create table bar;
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+---------------------------------------------------------------------
+--------------------------------------------+
| Table | Create Table                                                
+                                                                     
+                                                                     
+                                            |
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+---------------------------------------------------------------------
+--------------------------------------------+
| bar   | CREATE TABLE `bar` (
  `id` int(11) NOT NULL,
  `foo_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `foo_id_fk` (`foo_id`),
  CONSTRAINT `foo_id_fk` FOREIGN KEY (`foo_id`) REFERENCES `foo` (`id`
+)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------
+---------------------------------------------------------------------
+---------------------------------------------------------------------
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> drop table foo;
ERROR 1217 (23000): Cannot delete or update a parent row: a foreign ke
+y constraint fails
mysql> drop table bar;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table foo;
Query OK, 0 rows affected (0.00 sec)
[download]

As you can see, it looks like mysql silently ignores references in your SQL statements and doesn't create foreign keys. I would say it is rather nasty behaviour, if it doesn't accept your syntax it should throw an error.