Another approach you could take would be include the date in the filename, so instead of writing to vpnwarn.out all the time you could write to vpnwarn-11-12-2003.out (using POSIX::strftime() or something to format the filename). Then you could use a separate program to clean up these files when their number becomes too great.

It's strange that your files continue to grow if your code closely follows what you've posted here.