my @emails = (split(/\s*,\s*/, $email))[0..2];

##</code><code>##

my @emails = split(/\s*,\s*/, $email);

if (@emails > 3) {
   ... warn ...
}

# Keep only the first three.
splice(@emails, 3);

##</code><code>##

$email = join(', ', (split(/\s*,\s*/, $email))[0..2]);