Cheap caching with AutoDestruct

lazy.pl

# ta-da!
has expensive_value => (is => 'ro', lazy_build => 1);

sub _build_foo { ... something expensive ... }

not-so-lazy.pl

has expensive_value => (is => 'ro', lazy_build => 1);
sub _build_foo { ... something expensive ... }

has ev_birthday => (is => 'rw', clearer => 'clear_ev_birthday');

after _build_foo => sub { shift->ev_birthday(time()) };

before expensive_value => sub {
    my $self = shift @_;

    $self->clear_expensive_value
        if time() - $self->ev_birthday > 55*60; # 55 minutes, in seconds
};

very-lazy.pl

has expensive_value => (
    traits => [AutoDestruct],
    is => 'ro',
    lazy_build => 1,
    ttl => 55*60, # seconds
);
sub _build_foo { ... something expensive ... }

# somewhere else in your code, on an instance $foo
my $value = $foo->expensive_value; # first call, value generated!

# 30 minutes later
my $value = $foo->expensive_value; # time delta < ttl value; stored value used

# 30 minutes after that
my $value = $foo->expensive_value; # too old! value cleared; normal lazy gen follows

very-vs-not-so.pl

# 30 minutes after that
my $value = $foo->expensive_value; # too old! value cleared; normal lazy gen follows

# essentially:
my $value = do {
    $foo->clear_expensive_value
        if time() - $foo->ev_birthday > 55*60;
    $foo->expensive_value;
};