That works because you're not using strict and Perl is treating $#self->{op_stack} as if $self is an array reference. With strict, you would have been told that "Global symbol @self" requires an explicit package name at ...". That should have given you the information that you need to tweak the script a little.

$#{ $self->{ op_stack } }
[download]

This snippet will show you more:

package Foo;
use strict;

sub new
{
    my $class = shift;
    my $stuff =
    {
        a => [ qw/ Ovid is a schmuck / ]
    };
    bless $stuff, $class;
    return $stuff;
}

package main;
use base "Foo";

my $test = Foo->new;
use Data::Dumper;
print Dumper $test;
print $#{$test->{a}};
[download]

That prints '3', like it should.

Cheers,
Ovid

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