I think this error is related to Perl RT Ticket 39358 which is opened and discussed in http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2006-06/msg00276.html. The problem is that you cannot refer to the contents of the array within the comparator during an in-place sort, and it should be fixed in recent bleadperls, at least once dave_the_m gets around to it.