Just for fun, something a little more robust/checky. It can be made deeper to avoid obvious bounces and non-existent end points. See the doc: Email::Valid.

use Email::Valid;
use Carp qw( cluck );


sub join_email_lists {
    join ",", grep { Email::Valid->address($_) } @_;
}

sub join_email_lists_with_feedback {
    my @list;
    my @err;
    for my $address ( @_ )
    {
        if ( Email::Valid->address($address) )
        {
            push @list, $address;
        }
        else
        {
            push @err, $address;
        }
    }
    cluck "Problem address(es): ", join(", ", @err) if @err;
    join ",", @list;
}
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