Well if instead of the length of the strings you save the pos, getting the information from the first string is straightforward (and O(1) if you only need one character). It removes the need to copy the substrings altogether, since you can access them directly in the input string. It does look like you still get a significant gain when copying all the substrings into the array:

cmpthese -1,{
    a_copy=>q[
        @array = ();
        push @array, "$1" while $s =~ m[((.)\2*)]sg;
    ] ,
    a_pos=>q[
        @array = ();
        push @array, pos() while $s =~ m[(.)\1*]sg;
    ] ,
    
    b_cow=>q[ # There might be a COW mechanism because of the call to 
+substr
              @array = ();
              $s3 = " $s" ^ $s;; push @array, substr($s, pos(), length
+ $1) while $s3 =~ /(.\o{0}*)/gs
            ],
    b_copy=>q[ # Force copy, to avoid delayed penalty of COW
                @array = ();
                $s3 = " $s" ^ $s;; push @array, "".substr($s, pos(), l
+ength $1) while $s3 =~ /(.\o{0}*)/gs
             ],
    b_pos=>q[ 
              @array = ();
              $s3 = " $s" ^ $s;; push @array, pos() while $s3 =~ /.\o{
+0}*/gs
            ],
};;

__DATA__

         Rate a_copy  a_pos b_copy  b_cow  b_pos
a_copy  383/s     --   -20%   -49%   -61%   -80%
a_pos   478/s    25%     --   -36%   -51%   -75%
b_copy  747/s    95%    56%     --   -23%   -60%
b_cow   971/s   154%   103%    30%     --   -49%
b_pos  1888/s   393%   295%   153%    94%     --
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