# Returns error if last operation failed
sub error {
    my $self = shift;
    return $self->{'error'};
}

# ...

sub _error {
    my ($self, $message) = @_;
     
    $self->{'error'} = $message;           # <--(1)
    if (defined &{$self->{'error'}}) {     # <--(2)
        &{$self->{'error'}}($message);     # <--(3)
    } else {
        STDERR->print("Stripe Webhook Error: $message\n");
    }
}