# unsorted addresses start off in the @addresses array.
# sorted ones end up in the @recipients array.

foreach my $address (@addresses) {
    my ($name, $host) = split(/\@/, $address);
    # various checks on the validity of name and host, snipped
    push @{$host}, "$name\@$host";
    $frequency{$host}++;
}

foreach my $host (reverse sort { $frequency{$a} <=> $frequency{$b} } keys %frequency) {
    foreach my $address (@{$host}) {
        push @recipients, $address;
    }
    @{$host} = -1;
}