SQL Server uses Tabular Data Stream, which is essentially the same network protocol as Sybase uses. Thus, for the most native connection, you need
the DBD:
ybase module, in particular see the
notes on using it with SQL Server. There are two TDS libraries available, FreeTDS or Sybase's libraries.
Other options are
Active Data Object or
ODBC. ODBC is more of a generic standard; as I understand ADO is MS's latest and greatest. Personally have a look at what limitations each driver has, on CPAN you can see all the revisions and get an idea how mature each is. But most of all, find out which one is the easiest to get running with.