Marto put me on the right track. I was using minimal parameters according to DBI, but TOO minimal for DBD:Pg. Under 9.5.3 and activeperl 5.22 on Win10, it defaulted the final parameter of connect to {autocommit=>0}. Under 9.3 and perl 5.18 on Ubuntu, it got confused by the omission of the final parameter and just raised its default connect failure. So surprisingly enough, the logon problem was fixed by explicitly adding the {autocommit=>0}. I had missed it but got away with it for Windows but not for the new target Ubuntu platform (where it will eventually get hosted as a web service).