Agreed with the previous replies.

You missed a few calling conventions,

cmpthese( -1, {
    k1 => sub { $foo->do_nothing },
    k2 => sub { Foo->do_nothing  },
    k3 => sub { Foo::do_nothing  }, 
    k4 => sub { do_nothing       },      
    k5 => sub { &do_nothing      },      
    k6 => sub { do_nothing()     },
    k7 => sub { Foo::do_nothing()},
    k8 => sub { &Foo::do_nothing }, 
});
[download]

       Rate   k2   k1   k3   k4   k6   k7   k5   k8
k2 379516/s   -- -14% -31% -34% -35% -38% -48% -48%
k1 442514/s  17%   -- -20% -24% -24% -28% -39% -40%
k3 550801/s  45%  24%   --  -5%  -6% -10% -24% -25%
k4 579231/s  53%  31%   5%   --  -1%  -6% -20% -21%
k6 584645/s  54%  32%   6%   1%   --  -5% -20% -20%
k7 614031/s  62%  39%  11%   6%   5%   -- -16% -16%
k5 727406/s  92%  64%  32%  26%  24%  18%   --  -1%
k8 735179/s  94%  66%  33%  27%  26%  20%   1%   --
[download]