Blessed references and method receivers

The idea

A conventional Perl object is a reference associated with a package by bless. Method lookup uses that package, while the referenced data stores instance state. Calling a method with arrow syntax passes the invocant as the first argument, so a method commonly extracts $self before its remaining parameters. A class-method constructor receives the class name in that position.

A blessed hash is still accessible as a hash reference, so API discipline matters if callers could bypass invariants by changing fields directly. Keep public operations small and document what they may mutate. Test two independent instances and aliasing of one instance separately. Object-oriented syntax does not automatically copy data or prevent shared references between records.

use strict; use warnings;
{ package Counter;
  sub new { my ($class)=@_; bless {n=>0},$class }
  sub next { my ($self)=@_; return ++$self->{n} }
}
my $c=Counter->new();
print $c->next()," ",$c->next(),"\n";

1 2

The constructor blesses a fresh hash, and each method call receives that object as its first argument.