Perl developers report:
In order to prevent an algorithmic complexity attack against its hashing mechanism, perl will sometimes recalculate keys and redistribute the contents of a hash. This mechanism has made perl robust against attacks that have been demonstrated against other systems. Research by Yves Orton has recently uncovered a flaw in the rehashing code which can result in pathological behavior. This flaw could be exploited to carry out a denial of service attack against code that uses arbitrary user input as hash keys. Because using user-provided strings as hash keys is a very common operation, we urge users of perl to update their perl executable as soon as possible.