I suppose this is because the ampersand identifies the token as a function name, and so Perl doesn't have to spend any time disambiguating it -- but I'm surprised that that seems to be happening on the fly, at run-time.

Guess I'll be lubricating a few bottlenecks with ampersands...

#!/usr/bin/perl

# sub_speed.plx -- compare speed for calling sub

package Foo;

sub do_nothing { }

package main;
use strict;
use warnings;

use Benchmark qw( cmpthese );

my $foo = bless {}, 'Foo';

sub do_nothing { }

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      },      
});
[download]

k2 1456626/s   -- -17% -34% -36% -47%
k1 1745245/s  20%   -- -21% -23% -36%
k4 2210645/s  52%  27%   --  -3% -19%
k3 2277634/s  56%  31%   3%   -- -16%
k5 2723258/s  87%  56%  23%  20%   --
[download]