Well, if you are using mod_perl, you could install a handler for just about any stage in a request. See the mod_perl guide

On the other hand, if you just want to be notified when the access log is updated, you might try something like:

open LOG,"</var/log/httpd/access_log" or die "Cannot open log file: $!
+";

seek LOG,0,2;    # set to eof

while (1) {
    while (<LOG>) {
        print;  # or do something else...
    }
    sleep 1;    # this is 1 seconds, so not exactly realtime
    seek(LOG, 0, 1);  # reset eof flag
}
[download]

See also perldoc -f seek