UPDATE: this is bad code, I know. it's just a completely fake example I made up for this post. it's also deliberately contrived, just to make the real problem less apparent (and where's the fun, otherwise? :-). you have been warned. don't do this at home.

package SomeData;

# UPDATE: the first version of "new" doesn't seem to 
# work on some platform/version combination (and it's 
# way too evil anyway). but since this is not the main 
# issue, I will use a more "sane" version :-)

# sub new { bless \pop, shift }
sub new { my $self = pop; bless \$self, shift }

sub value { ${shift;} }

sub add {
    my($self, $type, $value) = @_;
    if($type eq 'number') { return $$self += $value; }
    if($type eq 'string') { return $$self .= $value; }
    die "only string or number allowed";
}

sub increment {
    my $self = shift;
    $self->add(number => 1);
}

package MyNumber;

use base qw( SomeData );

sub add {
    my($self, $value) = @_;
    die "not a number" unless $value =~ /^\d+$/;
    $$self += $value;
}

sub subtract {
    my($self, $value) = @_;
    die "not a number" unless $value =~ /^\d+$/;
    $$self -= $value;
}

package main;

my $answer = MyNumber->new(42);
print $answer->value(), "\n";

$answer->subtract(1);
print $answer->value(), "\n";

$answer->increment();
print $answer->value(), "\n";
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