If you squint just a little bit it's not that hard to move back and forth between this:

sub new
{
    my ($class) = @_;
    my $this = [];
    bless ($this, $class);
    
    $this->[PARENT] = undef;    
    $this->[PROGRESS] = 0;
    
    return $this;
}
[download]

and this:

sub new
{
    my ($class) = @_;
    my $this = {};
    bless ($this, $class);
    
    $this->{PARENT} = undef;    
    $this->{PROGRESS} = 0;
    
    return $this;
}
[download]

As others have already pointed out above, there are some performance perks to using a blessed array ref instead of a hash ref, but I think many of us back then felt that a hash ref was just more "natural" for an object (can the old far.... err ... C programmers like me say "struct"? I knew you could.)