If your aim is to create a class visible only to the enclosing class, then something along the following lines might suffice:

# File: Frobnicate.pm

use warnings;
use strict;

package Util;
.
.

package Frobnicate::Bar;
.
.

package Frobnicate::Foo;

our @ISA = qw/Frobnicate/;
.
.

package Frobnicate;
.
.

1;
[download]

Note that, within Frobnicate.pm, ...

• All classes (packages) are visible to each other.
• Only Frobnicate is accessible (& thus available as a superclass) externally.
• Frobnicate::Foo subclasses (and thus has a 'relationship' with) Frobnicate.
• Frobnicate::Bar has no 'relationship' with Frobnicate

Also, note that there is no need to use the subclass & utility classes where sub-classing behaviour is required.

Update:

The most apposite & absolutely correct) observations of both GrandFather & dsheroh have, between them, served to most ably demonstrate the...

• Voracity of my sig here on the monastry
• Correctness/validity of the (sometimes self-imposed) coding standards (&/or conventions) to which I have worked - my posting was purely experience based

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