Denial-of-Service attack. An attack that degrades the availability of a service. It was possible to intentionally trigger the degenerate case, and thus it could have been possible cause a service to spend a lot of time doing nothing.

Take for example a caching proxy written in Perl. By placing requests for carefully crafted urls, it could have been possible to negatively affect the performance of the proxy.

At no cost, the fix in 5.8.1 prevents this from happening.

( In no way does the OP have to worry about any of this )