Last time I uses DBD::ODBC with MS SQL Server 7 and 2000, only one statement could be active at any time, due to protocol limitiations of the MS SQL Server and its predecessor Sybase.

The protocol has been "extended" (or should I say botched?) since then to allow a complete different way for other SQL statements to execute in parallel to the main statement (I think this is called MARS). See DBD::ODBC::FAQ and http://www.easysoft.com/developer/languages/perl/multiple-active-statements.html for the ugly details.

Alexander