First, There are many logging modules out there, my favorite of which is Log::Log4perl. It would probably be easier if you just used one of these instead of writing your own.

Assuming this is not an option, there is a module called Cwd that comes standard with perl that will get you the current working directory. This will keep you from having to parse the directory out of $0. You might also want to turn warnings on, I found found that with warnings, it is much easier to find errors than without them.

Also it looks like you are trying to redirect the file error_log.txt to the httpd error log, but I am not seeing anywhere were error_log.txt is getting written to. Hope this helps.