use Benchmark qw( cmpthese );

$s = join'', map{ chr( 65+rand(26) ) x rand( 100 ) } 1 .. 1000;;

push @first, $1 while $s =~ /((.)\2*)/gs;
$s2 = " $s" ^ $s; # XORing the string with a shifted copy of itself, so that you have a series of 0s for identical characters
push @second, $1 while $s2 =~ /(.\o{0}*)/gs;

$\ = $/ x 2;
print pack "(A4)*", map length, @first;
print pack "(A4)*", map length, @second;

cmpthese -1,{
    a=>q[ 
        1 while $s =~ m[((?=(.))\2+)]g;
    ],
    b=>q[
        1 while $s =~ m[((.)\2*)]sg;
    ] ,
    c=>q[$s3 = " $s" ^ $s;; 1 while $s3 =~ /(.\o{0}*)/gs],
};;