How does $self get assigned to the same exact memory location every time a new object is created?

In normal usage, you don't get the same exact memory location every time a new object is created. Quite on the contrary: Every object has its own value for $self which is returned by the new() method as a reference to a new (!) hash. You might get the same memory location if you drop an object and then create another one, if Perl's memory management decides that way.

In your code example, $self is a lexical variable which gets a location in memory once, when the module's code is evaluated. The value of $self gets overwritten every time a new object is created, so the return value of kool_func_for_users changes with every new object - but then, it does so for all objects on that class. In effect, you have created a "class attribute" which has the name $self but in your case should actually be called $most_recent_object. There are valid use cases for these, and there's also a Moose module MooseX::ClassAttribute.

Here's a short demo (in which I've also fixed your attribute syntax by using parens instead of braces, and by using is and isa as intended by Moose):

use strict; use warnings; use 5.010; # saving some vertical space

package My::Demo;
use Moose;

has 'why_pass_self' => ( is => 'ro',
                         isa => 'Str',
                         default => 'dunno, is this ok?' );

my $self;

sub BUILD {
  my $s = shift;
  $self = $s;
}

sub _kool_func {
  say $self->why_pass_self;
}

sub kool_func_for_users {
  my $self = shift;
  $self->_kool_func;
}

#####################################################################
package main;

use Scalar::Util qw(refaddr);

my $o1 = My::Demo->new;
print "o1 ", refaddr($o1), " at says: ";
$o1->kool_func_for_users;

my $o2 = My::Demo->new(why_pass_self => "That's why!");
print "o2 ", refaddr($o2), " at says: ";
$o2->kool_func_for_users;
print "o1 ", refaddr($o1), " at says: ";
$o1->kool_func_for_users;
[download]

PS: I just noticed I've provided what Corion suggested:

Maybe consider just creating two objects of the same such class and watch what happens.

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