Wow! I tried your code and got the same thing!

DBI connect('TestCasesXP2K','',...) failed: [Microsoft][ODBC Driver Ma
+nager] Data source name not found and no default driver specified (SQ
+L-IM002)(DBD: db_login/SQLConnect err=-1) at C:\Daniel\perl\PerlMonks
+\dbitest.pl line 13
Can't call method "prepare" on an undefined value at C:\Daniel\perl\Pe
+rlMonks\db
itest.pl line 16.
[download]

I suspected that I might since I haven't set up an ODBC connection using the ODBC connection manager. If you set up and test your ODBC connection then you should be able to connect OK.