Perhaps something modeled after Net::Netmask's imask using Math::BigInt?

use Math::BigInt ();

sub ipv6_mask {
  my $bits = shift;
  my $mask =
    Math::BigInt->new( 2 )->bpow( 128 ) 
    - Math::BigInt->new( 2 )->bpow( 128 - $bits );
  return $mask->as_hex;
}
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Update: Ooop, you wanted an array of octets. return unpack( "a2"x16, $mask->as_hex ) will get you that.