sub new {
        my $this = shift;
        my $class = ref($this) || $this;



    # This part prevents a user from creating an
    # instance of the package, but still permits
    # subclasses to call the parent's new().

        my (undef, $filename, $line) = caller;
        $filename =~ s/^.*\///;
        die "Abstract class $class must " .
            "be subclassed at $filename " .
            "line $line.\n"
            if $class =~ /^Processor$/;



        bless $self, $class;
        return $self;
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The subclassed code's new looks like this:

sub new {
  my $this = shift;
  my $class = ref($this) || $this;
  my $self = $this->SUPER::new(@_);
  bless $self, $class;
  return $self;
}
[download]