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my $quad     = "[0-9a-fA-F]{0,4}";
my $ipv6addr = "(?:$quad:){2,7}$quad";
...
      or not /::/ and 7 != tr/:/:/;
    return ( 0 <= $mask and $mask <= 128 );
}

use warnings;
use strict;
...
2001:2:3:4:5:6:7:8/64
2001:2:3:4:5:6:7:8:9/96
2001:2:3:4::6:7:8:9/96

::                        0:0:0:0:0:0:0:0/128       yes
::/0                      0:0:0:0:0:0:0:0/0         yes
...
2001:2:3:4:5:6:7:8/64     2001:2:3:4:5:6:7:8/64     yes
2001:2:3:4:5:6:7:8:9/96                             no
2001:2:3:4::6:7:8:9/96                              no