perl access

I'm stuck on something stupid.

I'm trying to use perl to open up a database handle and I can't find the
right database dsn string.

my $data_source = "dbi:Pg:mydatabase";
my $username = "mydatebasereader";
my $auth = "secret";

my $dbh = DBI->connect($data_source, $username, $auth);



DBI connect('postfix','postfix',...) failed: missing "=" after "postfix"
in connection info string at /home/tallison/bin/perl.pl line 21

I've tried variations on the $data_source but I'm afraid I'm working on
a hit-n-miss process.

Can someone point me?