Oh, that's what you mean! Here's a sneaky way to do it:

#!/usr/bin/perl -w

use strict;

package Superclass;

use vars qw ( $AUTOLOAD );

my $classdata = qq|
{
    my \$self = shift;
    my \$invocations = 0;
    sub _increase {
        \$invocations++;
    }
    sub number {
        return \$invocations;
    }
}|;

sub new {
    my $class = shift;
    if ($class ne __PACKAGE__) {    # he he he
        eval qq|package $class; $classdata |;
    }
    my $self = {};
    bless($self, $class);
    $self->_increase();
    return $self;
}

eval $classdata;

package Subclass;

use vars qw ( @ISA );
@ISA = qw ( Superclass );

package main;

print Superclass->number();
my $one = Superclass->new();
print $one->number();
print Subclass->number();
my $two = Subclass->new();
print $two->number();
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If your superclass is in a different module, you can play around with the import subroutine and do it a bit more cleanly.

This technique has the unfortunate effect of making Subclass->number() return 1 before you create any instance of Subclass. (Inheritance... oh well.)