From the very first assignment to @emails, you have no idea how many, if any, elements it has.

A good general rule to follow is that when you don't know if there is a value, test it! ;)

The @emails = grep... statement doesn't need any input, so you're safe there.
The foreach... doesn't need @emails to have anything in it, so again, that's ok.

When you get down to the split(), you have a problem. If @newemails has no elements in it.. perl will complain, and $email and $domain will be undefined.

The simplest way to deal with this situation is to test if @newemails has any elements. Wrap any code that depends on it containing something, into that test:

if (scalar @newemails) {
    my ($email, $domain) = split(/\@/, $newemails[0]);
    mail($newemails[0], $domain);
}
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