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{ $string } + 1 },
});
__END__
# on my perl 5.8.7
Rate self string
self 156393/s -- -83%
string 927942/s 493% --
On another minor note, 0+$self yields the same result as the refaddr function.
Unless you overload arithmetic.