# this finds the links I do not want:
my @links = $mech->find_all_links(url_regex => qr/foo/);

# these (as well as some really bizarre attempts) don't work:
my @links = $mech->find_all_links(url_regex => !qr/foo/);
my @links = $mech->find_all_links(url_regex !~ qr/foo/);