my %EMail = (
    Category1 => { "Name One",   'some1@address.com', },
    Category2 => { "Name Two",   'some2@address.com', 
                   "Name Three", 'some3@address.com', },
    Category3 => { "Name Four",  'some4@address.com',
                   "Name Five",  'some5@address.com', 
                   "Name Six",   'some6@address.com',
                   "Name Seven", 'some7@address.com', 
                   "Name Eight", 'some8@address.com', },
);
@EMail{keys %{$EMail{$_}}}= values %{$EMail{$_}}  for keys %EMail;

sub GetEMailFor
{
    my @addresses= ();
    for my $dest (@_) {
        my $addr= $EMail{$dest}
          or  die "No such e-mail destination: $dest\n";
        push @addresses, ref($addr) ? values %$addr : $addr;
    }
    return join ',', reverse @addresses;
}