One way to do it would be to use Tie::File. This will treat the log like an array. You can then shift off one line before using push to add a line. I do have to caution you that this will slow things down and may use quite a bit of memory.

Another solution involve writing to a logging daemon instead of directly to the log. The daemon can then monitor size and buffer input while rolling a log. I believe you might find something on CPAN for this, but am not sure.

Update: After having a discussion with borisz via /msg it appears that rotatelogs is very similar to my second solution. It does not require Apache. The difference is it uses time based rotation instead of size. This may not be what you want but if you can't find a canned solution on CPAN it something.