I found the earlier answers rather complex. It's easy to explain by showing what happens if you don't.

{
   package MyClass;
   sub new  { my ($class) = @_;
              my $self = {};
              return $self; }
   sub test { print("Hi!"); }
}

my $o = MyClass->new();
$o->test();
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outputs

Can't call method "test" on unblessed reference
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Because the hash has not been associated with a package, Perl doesn't know which test you meant to call. bless is the means by which you create this association.

{
   package MyClass;
   sub new  { my ($class) = @_;
              my $self = {};
              bless($self, $class);  # Added
              return $self; }
   sub test { print("Hi!"); }
}

my $o = MyClass->new();
$o->test();
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outputs

Hi!
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So how do Java and C++ work without something like bless? It's a combination of having constructors and of the finer grain of their type systems.