package Particle;

...

sub newMoveableParticle {
    my $class = shift;
    my $p = $class->new( @_ );
    $p->set( 'is_moveable', 1 );
    return $p;
}

sub newStationaryParticle {
    my $class = shift;
    my $p = $class->new( @_ );
    $p->set( 'is_moveable', 0 );
    return $p;
}

##</code><code>##

$REGISTRY{$self} = $self;

##</code><code>##

DESTROY {
    my $self = shift;
    delete $REGISTRY{$self};
}