You can use -e to check if a file exists. Something like

if (-e "./log.conf") {

     Log::Log4perl->init("./log.conf");

} elsif (-e "../log.conf") {
 
     Log::Log4perl->init("../log.conf");

} else {

     Log::Log4perl->init("/etc/log.conf");

}
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Code is untested and I've been forced to use ASP for the last month so it could be totally wrong. I'm sure there is a more elegant solution too.