oh, I see, Class::SingletonProxy doesn't work that way, you don't have to use it as the base for your classes.

Suppose you already have a Car class...

package Car;
our @ISA = qw(Vehicle);
# nothing related to Class::SingletonProxy goes here!

sub wheels { 4 }
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... and so you want to create a singleton for it:

package MyCar;
use base 'Class::SingletonProxy';

sub SINGLETON { Car->new('Ferrary', 'F40', 'red') }
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and then you can access the singleton from your scripts as...

MyCar->go_supermarket(speed => 200);
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Notice that the Car class remains untouched!