Does your error_log look like this?

10169 Apache::DBI PerlChildInitHandler
10169 Apache::DBI skipping connection cache during server startup
Database handle destroyed without explicit disconnect at
/usr/lib/perl5/site_perl/5.6.1/Apache/DBI.pm line 29.

If so, you are trying to open a database connection in the parent httpd process. If you do, the children will each get a copy of this handle, causing clashes when the handle is used by two processes at the same time. Each child must have its own unique connection handle.

To avoid this problem, Apache::DBI checks whether it is called during server startup. If so, the module skips the connection cache and returns immediately without a database handle.

You must use the Apache::DBI->connect_on_init( ) method (see the next section) in the startup file to preopen a connection before the child processes are spawned.