choroba helped with the immediate problem, but maybe an example of common usage will help:

use strict;
use warnings;

package Pendulum;

sub new {
    my ($class, %params) = @_;

    # Do interesting parameter validation here

    my $self = bless \%params, $class;

    # and other set up here. At this point you can call derived class 
+methods
    # as part of setup if you want using $self->methodName();

    return $self;
}


package main;

my $pit = Pendulum->new(Hz => 0.5);
[download]

Note in particular Pendulum->new(...) used to call the constructor