# Broken version:
next LASTLOG unless exists $user_by_uid{$uid}->{$host};

# Data::Dumper output when $uid is not in the hash:
$user_by_uid = {
                 '93688' => {},
                 '58684' => {},
                 '58017' => {},
                };

# Fixed version:
next LASTLOG unless exists $user_by_uid{$uid} && exists $user_by_uid{$uid}->{$host};