I'm not sure how an Ingres database works but if it is anything like Oracle your 'mydb' is a reference to a database defined in your current system on some host.

In Oracle this identifier is called a tnsname and is usually defined in a file called tnsnames.ora. That file lists the tnsnames and the host, db name (SID), and port that are needed to connect.

For example: when I connect to our development oracle database I use "DBI:Oracle:dev" as the database name. In our tnsnames file on the machine I'm working on there is an entry defining "dev" as a database called "MWH" located on the host "cosmora01d" and port "1521".

So the question is to you, what is mydb actually referring to and is there something similar in Ingres

Update:

I just looked at the CPAN DBD::Ingres pod...
http://search.cpan.org/~htoug/DBD-Ingres-0.51/Ingres.pm
and it refers to this syntax...
  DBI:Ingres:thatnode::thisdb;-xw -l
to connect to a Ingres/Net database