On the other hand, who says we have to use native numbers!

use strict;
use warnings;

use Array::IntSpan qw( );
use Math::BigInt   qw( );
use Socket         qw( AF_INET6 inet_pton );

sub ip6_to_num {
   return Math::BigInt->new(
      '0x' . unpack 'H32', inet_pton AF_INET6, $_[0]
   );
}

my $span = Array::IntSpan->new(
   [
      ip6_to_num('2001:0db8:85a3:0000:0000:0000:0000:0000'),
      ip6_to_num('2001:0db8:85a3:ffff:ffff:ffff:ffff:ffff'),
      'Some Network',
   ]
);

for my $addr (qw(
   2001:db8:85a3::8a2e:370:7334
   2001:dc8:85a3::8a2e:370:7334
)) {
   my $network = $span->lookup(ip6_to_num($addr));
   printf("Address %s is %s\n",
      $addr,
      $network ? "present in $network" : 'absent',
   );
}
[download]

Address 2001:db8:85a3::8a2e:370:7334 is present in Some Network
Address 2001:dc8:85a3::8a2e:370:7334 is absent
[download]