The performance of the blessed hash case is dependent on the length of the keys used in the hash: The longer the key, the more time it takes!

You're right, but this isn't the reason that using $self is so much slower. Stringification of references is just slow:

#! /usr/bin/perl

use strict;
use warnings;
use Benchmark qw( cmpthese );

my $self = bless {}, 'SomeClass';
my $string = "$self";

my %a = ( $self => 0 );
my %b = ( $string => 0 );

cmpthese(-1, {
    self    =>  sub { $a{ $self } = $a{ $self } + 1 },
    string  =>  sub { $b{ $string } = $b{ $string } + 1 },
});

__END__

# on my perl 5.8.7
           Rate   self string
self   156393/s     --   -83%
string 927942/s   493%     --
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On another minor note, 0+$self yields the same result as the refaddr function.

Unless you overload arithmetic.