I'd guess that you have run into the problem where a MySQL upgrade has moved the socket to /tmp/mysql.sock.

To clarify kubrat's response, here is an example based on my own setup:

my $dbh=DBI->connect('dbi:mysql:database=foo; host=localhost; mysql_so
+cket=/var/run/mysqld/mysqld.sock', 'myuser', 'myuserpass');
[download]

You can also set an environment variable before making your connection (without specifying the socket):

$ENV{MYSQL_UNIX_PORT}='/var/run/mysqld/mysqld.sock';

my $dbh=DBI->connect('dbi:mysql:database=foo; host=localhost', 'myuser
+', 'myuserpass');
[download]

Putting this in as a DBI connection parameter is much neater though.

Hope this helps.