...
# determine v4 or v6, nothing selected - then both
if (!defined($opt{ipv4}) && !defined($opt{ipv6})) {
    $opt{ipv4} = $opt{ipv6} = 1
}

...

# server (select between v4 and v6 if both exist)
my $server = IO::Select->new();

# v4 server
if ($opt{ipv4}) {
    my $server4 = IO::Socket::INET->new(
                                        Proto => 'tcp',
                                        LocalPort => $opt{port},
                                        Listen => SOMAXCONN,
                                        Reuse => 1
                                       ) or die "$0:  Unable to create IPv4 server: $!\n";
    $server->add($server4)
}

# v6 server
if ($opt{ipv6}) {
    my $server6 = IO::Socket::INET6->new(
                                         Proto => 'tcp',
                                         LocalPort => $opt{port},
                                         Listen => SOMAXCONN,
                                         Reuse => 1
                                        ) or die "$0:  Unable to create IPv6 server: $!\n";
    $server->add($server6)
}

...

# determine which server has data
while (my @servers = $server->can_read) {
    # loop servers
    for my $svr (@servers) {
        # read from v4 or v6 server that has data
        while(my $client = $svr->accept()) {

#DO STUFF

        }
    }
}