Perl/LJ Question

So.. I'm trying to understand both Perl and LJ. Looking at talkpost_do, I see the following set of commands:

 return LJ::server_down_html() if ($LJ::SERVER_DOWN);

 my $dbs = LJ::get_dbs();
 my $dbh = $dbs->{'dbh'};
 my $dbr = $dbs->{'reader'};

What the heck is LJ::, and where did it come from. How does Perl know what it is?

Thanks for your help in helping me to further understand. :)