my $ONE_TRUE_INSTANCE = __PACKAGE__->new;
sub new {
    my ($class, %args) = @_;

    return $ONE_TRUE_INSTANCE
        if ($ONE_TRUE_INSTANCE and $class eq __PACKAGE__ and !%args);

    # normal constructor here
    ...
};

##</code><code>##

package Defaulton;

use Moo;
has foo => is => ro => default => sub { 42 };
my $INST = __PACKAGE__->new;

around new => sub {
    my ($orig, $class, @args) = @_;

    return $INST if $INST and $class eq __PACKAGE__ and !@args;

    return $class->$orig(@args);
};

1;