Could you provide a simple example of how one would use a class variable in Perl?

Sure!

#! perl
use strict;
use warnings;

package Widget
{
    my $count = 0;

    sub new
    {
        my ($class, @args) = @_;
        my  %self = @args;
        ++$count;
        return bless \%self, $class;
    }

    sub DESTROY   {      --$count; }
    sub get_count { return $count; }
}

package main;

my $w1 = Widget->new(name => 'foo', value => 42);
my $w2 = Widget->new(name => 'bar', value =>  7);
my $w3 = Widget->new(name => 'baz', value => 12);
printf "Now there are %d Widgets\n", Widget::get_count;
undef $w1;
printf "Now there are %d Widgets\n", Widget::get_count;
[download]

Output:

15:52 >perl 856_SoPW.pl
Now there are 3 Widgets
Now there are 2 Widgets

15:52 >
[download]

Is there any other way, using the code I wrote, to access the value of an instance variable with a method that did not receive the variable as a parameter?

Not that I can think of. Anyway, that’s not how it’s supposed to work in Perl.

a class variable could be defined outside the constructor but still as a lexically scoped, 'my' variable? What about using 'our'?

Using our creates a package variable, which can then be exported to other packages, or accessed directly by other packages using the $Widget::count syntax. Using my creates a lexical variable, which is scoped to the package. The rule is: use a lexical (my) variable unless you have a good reason to make it a package/global (our) variable. Update (Jan 30, 2014): See Ovid’s classic post 'our' is not 'my'.

Hope that helps,

our %error_dictionary = ...;
our %cached_templates = ...;
sub get_template { .... check cache or store in cache ... }
sub get_error { $error_dictionary{...} ... }
[download]

If these were, my lexical, then its hard to peek inside for folks that need to ...

Being our globals, anyone who wants to peek can do so at their own risk, an know to stick with the official api for officialness

one or the other, it doesn't really matter, this is the least important type of implementation detail... but you can't go wrong with openness (our our our)

https://metacpan.org/module/DROLSKY/perl-5.15.6/pod/perlootut.pod, chromatics free book Modern Perl, Moo, Re^3: High level OOP query (internal data)