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;
}