AUTOLOAD is a subroutine with a special function: If you try to invoke a non-existing subroutine, you usually get an error. However, if the package has a subroutine called AUTOLOAD, the subroutine is invoked instead. The name of the subroutine that was requested is in the package global $AUTOLOAD.
in reply to wat ::DESTROY will do
This AUTOLOAD subroutine can then do any number of useful things, such as creating the missing subroutine. This is a common pattern in OO Perl, where accessors for instance variables are created that way.
When trying to find an AUTOLOAD, Perl starts to look in the package that contains the subroutine you asked for, but it also digs its way through @ISA. So, if you define AUTOLOAD in the UNIVERSAL package (the base class for all Perl objects), this AUTOLOAD gets applied for all packages (unless they have their own AUTOLOAD).
One thing your AUTOLOAD probably does not want to handle is the DESTROY subroutine, which gets called as a destructor method when objects (blessed references) get garbage-collected. This is the reason why above code checks if the requested subroutine name is DESTROY. It will only do its magic for other subroutines.
Check out perltoot for details on AUTOLOAD.