Don't know if it will trigger the necessary loading you're looking for, but the code below will load a page and scroll down 188 lines. Once you have the window object you can use any method
listed here. There are a handful of methods that allow you to scroll based on different parameters.
My guess that if this is going to work as you hope, you'll have to scroll incrementally and not just pop to the bottom of the document.
use WWW::Mechanize::Firefox;
my $mech = WWW::Mechanize::Firefox->new(tab => 'current');
$mech->get('http://search.cpan.org/~corion/WWW-Mechanize-Firefox/lib/W
+WW/Mechanize/Firefox.pm');
my ($window,$type) = $mech->eval('window');
$window->scrollByLines(188);