Perl DBI connection to Postgres

I'm tyring to use the Perl::DBI module to connect to a postgreSQL database but
it doesn't seem to like it?

What is the connection statement?! Have used:

use DBI;
$dbh=DBI->connect('DBI:Pg:user_name', 'user_name', 'password')

but this returns a message that the connection failed due to a missing "=" after
the user_name?

Any suggestions?