Oh, that's what I didn't understand. I changed my $ua to $mech and the images came booming through bright and clear. Thank you very much!!
Update: Sorry, to be clear, for the record:
my $mech = WWW::Mechanize->new(
cookie_jar => {
file => 'cookies.txt',
ignore_discard => 1,
autosave => 1
}
);
$mech->get($add);
my @links = $mech->find_all_images(url_regex => qr/jpg/);
my $countphotos;
for my $img (@links) {
$countphotos++;
my $photofile ="photo" . $countphotos . '.jpg';
my $a = $img -> url_abs;
$mech->mirror( $a, $photofile );
}