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.

In reply to Re (tilly) 1: __PACKAGE__-compile .. what in the world is this? by tilly
in thread __PACKAGE__-compile .. what in the world is this? by princepawn

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.