in reply to modifying sub routine behaviour
AOP is a little bit more than wrapping subroutines :-)
For those who care the Introduction to Aspect Oriented Programming with AspectJ has a nice overview. There are also the Aspect modules on CPAN.
Indeed, AspectJ is a prime resource for information on AOP at all levels. (adrianh's link is outdated.) The current Aspect dist for Perl is Aspect-0.12. Lots more good information under Aspect-Oriented Programming: Couple of Short Examples