Others have answered the "what". Here is the where.

__PACKAGE__ is defined in perldata.

After you understand that __PACKAGE__ will be replaced with the current package, the workings of the method call are described in perlobj and elsewhere.

To discover this yourself, along with the other references to __PACKAGE__, try:

perldoc -l perl
[download]

to find what directory perl.pod is in, cd to that directory, and then grep for __PACKAGE__.

UPDATE
Oops. Kanji pointed out that his post did answer the where. But the idea of using grep is still worth knowing about as a way to find these things out when you don't know where to look first.