Mine

my $dbh = DBI->connect("dbi:Pg:dbname=musicbrainz_db", "musicbrainz_user");

DBI connect('dbname=musicbrainz_db','musicbrainz_user',...) failed: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?
 at dbtest.pl line 7

Works fine when I run the following at command line:

psql -U musicbrainz_user -d musicbrainz_db