package Baz;
           BEGIN {
               require Foo;
               require Bar;
               push @ISA, qw(Foo Bar);
           }

##</code><code>##

my $object = MyClass->new;
print $object->name;
print $object->{'name'};

##</code><code>##

package MyClass;
use strict;
use vars qw/$MUSTHAVEONLYONEINSTANCE/;
$MUSTHAVEONLYONEINSTANCE = 0;
...

sub new {
   return undef if $MUSTHAVEONLYONEINSTANCE;
   $MUSTHAVEONLYONEINSTANCE = 1;
   ...
}

sub DESTROY {
   $MUSTHAVEONLYONEINSTANCE = 0;
}