my $quad     = "[0-9a-fA-F]{0,4}";
my $ipv6addr = "(?:$quad:){2,7}$quad";

sub is_ipv6 {
    local $_ = $_[0];
    return 0
      unless m{^$ipv6addr(?:/(\d+))?$};
    my $mask = defined($1) ? $1 : 128;
    # in 5.10: my $mask = $1 // 128;
    return 0
      if /:::/
      or /::.*::/
      or not /::/ and 7 != tr/:/:/;
    return ( 0 <= $mask and $mask <= 128 );
}