use WWW::Mechanize::Firefox;
my $mech = WWW::Mechanize::Firefox->new(tab => 'current');
$mech->get('http://search.cpan.org/~corion/WWW-Mechanize-Firefox/lib/WWW/Mechanize/Firefox.pm');
my ($window,$type) = $mech->eval('window');
$window->scrollByLines(188);