Use a cipher as an "electronic code book". For example, fire up RC4 (which is easy to write in Perl, and allows the output to be the same length as the input for any size). Clone the RC4 object's state, since it doesn't have an ECB mode normally.

Now, encode your counter, a 3-byte integer.

Reset your RC4 object's state back to the beginning, then increment your counter and repeat. Each input gives a different output, and you can maintain a simple high-water mark.

A bijective hash function will do the same thing, but it would be harder to prevent the underlying pseudorandom sequence from being discovered by watching the output for a while.

—John