I think that $entry may not be getting initialized properly. I tested your regex with the \@ and it appears to work fine. eg:

$entry = 'a.address@host.com';
if ($entry !~ /^([a-zA-Z0-9\@_.]*)$/) {
    print "Doh!\n";
    exit;
}
print "Found it\n";
[download]

prints "Found it!"