thx for pointing out. what about

'regexpad' => sub 
{
  # padding the string 
  my $padding = 8 - length($str%8) if length($str%8); #has to be 8 - m
+odulo not modulo, thx johngg
  $str .= "x" x $padding;

  # dividing the string in parts of 8 chars
  my @arr = $str =~ /(........)/g;

  #remove padding
  $arr[-1] = substr($arr[-1],-$padding);
}
[download]

although its a bit slower than the other 2 regex solutions

               Rate split_pos grep_split substr_map regexpad regexo re
+gex substr_loop unpack
split_pos    5841/s        --       -64%       -70%     -75%   -76%  -
+76%        -79%   -80%
grep_split  16129/s      176%         --       -17%     -30%   -33%  -
+34%        -42%   -45%
substr_map  19531/s      234%        21%         --     -15%   -19%  -
+20%        -30%   -33%
regexpad    22936/s      293%        42%        17%       --    -5%   
+-6%        -17%   -22%
regexo      24038/s      312%        49%        23%       5%     --   
+-1%        -13%   -18%
regex       24272/s      316%        50%        24%       6%     1%   
+ --        -13%   -17%
substr_loop 27778/s      376%        72%        42%      21%    16%   
+14%          --    -5%
unpack      29240/s      401%        81%        50%      27%    22%   
+20%          5%     --
[download]

Edit: corrected padding