Your _cube can be replaced with:

sub _cube { $_[0] ** 3 }
[download]

but _cuberoot is a bit tougher if it might have negative bases:

sub _cuberoot {
  my $res = abs($_[0])**(1/3);
  $_[0] < 0 ? -$res : $res;
}
[download]

-- Randal L. Schwartz, Perl hacker