I'm not familiar with the Microsoft product but I guess it supports JDBC connections drivers. If it does, enable the JDBC service on the server side then use DBD::JDBC on the client (Unix) side. In order to use DBD::JDBC you need Java to run a middleware program together with the drivers from Microsoft (not necessarily on the same computer with the client application). Just my 2c.

Update: Another idea: run DBI::ProxyServer on a windows machine with MS SQL ODBC drivers and use DBD::Proxy on the Unix client.

Update 2: Mixed up client-server stuff...