I don't know what PBP has to say on the matter, as its one of the few Perl books I haven't bought. But...

sub new {
    my $self = shift;
    bless \( my $scalar ), ref $self || $self;
}
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Is better written as

sub new {
    my $class = shift;
    bless \( my $scalar ), $class;
}
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You don't normally get a blessed reference passed into new(). If you're using my $foo = Object::Foo->new, or something similar, its the name of the class (Object::Foo in my example) that's passed into new(). So, my $class = shift; is more logical.