package Child;
use parent 'Parent';
sub new {
    my ($class, $params) = @_;

    # call parent's constructor first with the given parameters
    # the parent should ideally filter out the parameters
    # that do not concern it and ignore them rather than complaining about illegal params
    my $self = $class->SUPER::new($params);
    # at this point $self has all Parent's initialisation
    # (whatever was done and set in its constructor).

    # Now do local initialisation on $self which is Parent's self (a {} in most cases)
    ...
    $self->{child_extra_property_1} = 12; # for example
    ...

    # and bless to Child - actually that's a re-bless because calling
    # Parent's constructor blessed $self to be an object of class 'Parent'
    # so now make it of class 'Child'
    bless $self => $class;
    return $self;
}