LukeyBoy is right, old versions of apache include a version of expat which conflicts with the one used by XML::Parser (and thus by XML::Simple).

The best explanation I have found on how to solve the problem is the one in the AxKit FAQ:

First of all, to find out if this is your problem, execute the command: strings /path/to/apache/bin/httpd | grep -i XML. If there are any results at all then you are going to see these segfaults until you recompile Apache (and probably mod_perl too)

To compile Apache without expat, simply supply the option:
RULE_EXPAT=NO

to ./configure when you build Apache. Alternatively if you build Apache with mod_perl (with mod_perl's DO_HTTPD option) it will supply this option automatically for you.