In
MT/ObjectDriver/DBI/mysql.pm

version

# Copyright 2001-2003 Six Apart. This code cannot be redistributed without
# permission from www.movabletype.org.
#
# $Id: mysql.pm,v 1.24 2003/02/12 00:15:08 btrott Exp $

in

sub init {}

there is no check for the DBPort config element.

A line similar to

$dsn .= ';port=' . $cfg->DBPort if $cfg->DBPort;

should be added after the line

$dsn .= ';mysql_socket=' . $cfg->DBSocket if $cfg->DBSocket;