http://qs1969.pair.com?node_id=11134355


in reply to Re^5: DBI placeholders for spatial data
in thread DBI placeholders for spatial data

I think this is the important part of the documentation

As a convenience, if the $data_source parameter is undefined or empty, the DBI will substitute the value of the environment variable DBI_DSN. If just the driver_name part is empty (i.e., the $data_source prefix is "dbi::"), the environment variable DBI_DRIVER is used. If neither variable is set, then connect dies.

I guess that the environment variable DBI_DRIVER is set to mysql as that is the RDBMS installed on the server.