Since it turns out to be stack manipulation providing the savings, the cases where I'm going to be able to take advantage of the ampersand are few and far between.

cmpthese( -1, {
    k1 => sub { $foo->do_nothing(1) },
    k2 => sub { Foo->do_nothing(1)  },
    k3 => sub { Foo::do_nothing(1)  }, 
    k4 => sub { do_nothing(1)       },      
    k5 => sub { &do_nothing(1)      },      
});
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        Rate   k2   k1   k5   k3   k4
k2 1380806/s   -- -14% -34% -35% -37%
k1 1610612/s  17%   -- -23% -25% -26%
k5 2103412/s  52%  31%   --  -1%  -4%
k3 2133262/s  54%  32%   1%   --  -2%
k4 2181254/s  58%  35%   4%   2%   --
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