The string '123456789' is very short. Try testing with a very long string:

our $s = '123456789' x 100_000;
cmpthese -1, {
 a=>q[ my ($a,$b) = $s =~  m/(23)[^8]+(8)/g; ],
 b=>q[ my ($a,$b) = $s =~  m/(23)[^8]+(8)/; ],
 c=>q[ my ($a) = $s =~  m/(23)/g ],
 d=>q[ my ($a) = $s =~  m/(23)/; ],
};
[download]

Outputs:

    Rate      a      c      d      b
a 7.65/s     --   -34%   -99%   -99%
c 11.7/s    52%     --   -99%   -99%
d 1336/s 17371% 11358%     --    -3%
b 1380/s 17950% 11738%     3%     --
[download]