local $_; # should always do this unless you know why you don't want it

my %id_cache;
open (IMAGE_LIST, $image_list); # you didn't need quotes here..
while(<IMAGE_LIST>) {
    $id_cache{substr($_,1)} = substr($_,0,1) eq 'Y';
}
close (IMAGE_LIST);

open (IMAGE_LIST, ">>$image_list"); # although you do here
for (@thumb_id_list) {
    unless(exists $id_cache[$_]) {
        # IDs that didn't appear in the file won't exist in the hash
	# therefor we have to connect to partner website for them
        $id_cache[$_] = &check_image($session,$_);
        print IMAGE_LIST $id_cache[$_] ? "Y$_\n" : "N$_\n";
    }

    # $id_cache[$_] is now defined,
    # regardless of whether it was in the file or not
    push (@display_thumbs,$_) if $id_cache[$_];
}
close (IMAGE_LIST);