There are two things off-hand that I'd try:

first enable tracing. the DBI docs have a section on tracing, and it has helped me to diagnose DBD weirdnesses. Use a high tracing level.

If that doesn't help, try newer version of your modules (and possibly even perl). If that's not easy on your current machine, try to do it on a different machine, just to see if solves your problem. After all your Perl is 3 major versions out of date, and DBD::DB2 has had 6 releases since your version.