Interestingly, if you throw a literal value in the mix, as done below, you will find that the literal is even usually a hair slower than a my variable. I'd be very curious to know why that is: 
#!/usr/bin/perl -w
use strict;
use vars qw(*const_alias $const_local);
use Benchmark qw(cmpthese);

local *const_alias = \100;
my    $const_ref = \100;
local $const_local = 100;
my    $const_my = 100;
sub CONST_SUB () { 100 };
use constant CONST_MOD => 100;

print "mod[".CONST_MOD."]\nsub[".CONST_SUB."]\n"
  ."my[$const_my]\nlocal[$const_local]\n"
  ."ref[$$const_ref]\nalias[$const_alias]\n";

# run iteratively within the sub to get a less skewed
# result - that's my theory anyway :-)
my $iter = 500;
my $t;

cmpthese (-5,
            {
              CMod   => sub { $t = CONST_MOD     for 1..$iter },
              CSub   => sub { $t = CONST_SUB     for 1..$iter },
              CMy    => sub { $t = $const_my     for 1..$iter },
              CLocal => sub { $t = $const_local  for 1..$iter },
              CRef   => sub { $t = $$const_ref   for 1..$iter },
              CAlias => sub { $t = $const_alias  for 1..$iter },
              CLit   => sub { $t = 100           for 1..$iter },
            });

### RESULTS (repeatable this time :) ###

         Rate   CRef CAlias CLocal   CSub   CLit   CMod    CMy
CRef   3735/s     --    -9%    -9%   -10%   -11%   -11%   -13%
CAlias 4088/s     9%     --    -0%    -2%    -3%    -3%    -5%
CLocal 4088/s     9%     0%     --    -2%    -3%    -3%    -5%
CSub   4172/s    12%     2%     2%     --    -1%    -1%    -3%
CLit   4208/s    13%     3%     3%     1%     --    -0%    -2%
CMod   4210/s    13%     3%     3%     1%     0%     --    -2%
CMy    4293/s    15%     5%     5%     3%     2%     2%     --

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   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

In reply to Re(4): my $var; vs. use constant VAR = ''; by MeowChow
in thread my $var = ''; vs. use constant VAR => ''; by antihero

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