You cannot determine if he's using old DBD::mysql or libmysql.
The obvious question is: Are the C libraries thread safe? In the case of MySQL the answer is "mostly" and, in theory, you should be able to get a "yes", if the C library is compiled for being thread safe (By default it isn't.) by passing the option -with-thread-safe-client to configure. See the section on *How to make a threadsafe client* in the manual.

In reply to Re^3: share MySQL resources among threads by Anonymous Monk
in thread share MySQL resources among threads by jeanluca

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