If you're on Windows, then yes, as ikegami says, it's DBD::ODBC.

Just for completeness, let me refer you to MSSQL::DBlib and MSSQL::Sqllib by Erland Sommarskog. These work directly with SQL Server, and don't require either ODBC or FreeTDS. On the other hand, they don't work with DBI; to use them, you'll have to learn a new interface (fully documented in the distributions).