Argh! The m-capture regex has a bug: the final ? should be a * (and the corresponding ? in s-capture2). Naturally, this slows things down a bit (and I can't update the original answer).
Corrected code:

  's-capture2'    => sub { $_=$f; s/^\s*(\S+(?:\s+\S+)*)?\s*$/$1/; },
  'm-capture'     => sub { $_=$f; ($_) = /(\S+(?:\s+\S+)*)/;    },
[download]

New results (on a different machine):

                  Rate Regexp-Common s-capture one-s/// s-capture2 m-c
+apture two-s///
Regexp-Common   6629/s            --      -89%     -90%       -91%    
+  -94%     -97%
s-capture      62950/s          850%        --      -1%       -18%    
+  -41%     -71%
one-s///       63621/s          860%        1%       --       -17%    
+  -41%     -71%
s-capture2     77016/s         1062%       22%      21%         --    
+  -28%     -65%
m-capture     107187/s         1517%       70%      68%        39%    
+    --     -51%
two-s///      219320/s         3209%      248%     245%       185%    
+  105%       --
[download]