The PrintWarn attribute used to default to the same value as $^W. In other words, if you ran perl with -w then would be on.
See https://metacpan.org/module/TIMB/DBI-1.627/DBI.pm#PrintWarn That was reasonable back in early 2004 when PrintWarn was added to the DBI. I'm planning to change PrintWarn to always default on. There's a small risk that this change will cause extra warnings to be logged from your application if it's doing something that triggers the driver to record warning states (via the set_err method). I've uploaded DBI-1.628_50 as a trial release of DBI-1.629 so you can test it *with your application code* (not just make test) and report back. Please report back whether you have extra warnings logged *OR NOT* and don't forget to mention what driver you're using. Thanks! Tim. p.s. DBI-1.628_50 will install as $DBI::VERSION 1.629. Changes: https://metacpan.org/changes/release/TIMB/DBI-1.628_50#Changes-in-DBI-1.629