use strict;
use warnings;
use Regexp::Common 'net';
use Carp;

sub to_ipv4 {
    my $v6 = shift;

    unless ($v6 =~ /$RE{net}{IPv6}{-keep}/) {
        carp "'$v6' is not a valid IPv6 address";
        return;
    }

    unless ($2 eq '2002') {
        carp "'$v6' is not a 6to4 IPv6 address";
        return;
    };

    my @octets;
    my @parts = ($4, $3);
    for my $part (@parts) {
        unshift @octets, hex substr ($part, -2, 2, '') for 1, 2;
    }
    return join '.', @octets;
}

# Example usage
my $v4 = to_ipv4 ($ARGV[0]) // 'invalid';
print "$v4\n";