Even more intriguing. The slowest has hardly changed, but the previously faster ones have slowed markedly.

C:\test\perl-5.14.0-RC1>perl
use Benchmark qw[ cmpthese ];;
print $];;

$str = '123456789';
cmpthese -1, {
 a=>q[ my ($a,$b) = $str =~  m/(23)[^8]+(8)/g; ],
 b=>q[ my ($a,$b) = $str =~  m/(23)[^8]+(8)/; ],
 c=>q[ my ($a) = $str =~  m/(23)/g ],
 d=>q[ my ($a) = $str =~  m/(23)/; ],
};;
^Z
5.014000      Rate    b    a    d    c
b 363518/s   -- -17% -35% -46%
a 435446/s  20%   -- -22% -35%
d 555991/s  53%  28%   -- -17%
c 668598/s  84%  54%  20%   --
[download]

But still, 20% is not to be sneezed at.