Always free?

[jbn@forestfield.org (J.B. Nicholson-Owens)]

Mark Rafn wrote:
> It [Perl's copyright holders] can't retroactively change licenses.
> There will always be a free Perl.

It is my understanding that licensees (generally) haven't been given any
consideration in exchange for the software, so a license can be revoked by
the licensor at any time.  I, for example, never paid anyone for the license
to my copy of Perl.  So my rights under that license can be revoked if the
copyright holder wants to change the license conditions or revoke the
license and there's nothing I can do about that.

As I understand things this is one of the risks of using licenses (as
opposed to contracts): I copy, modify, and distribute[1] Perl under the risk
that my ability to copy, modify, and distribute Perl can be taken away.

Furthermore, my understanding is that this principle of legal
"consideration" trumps any wording in the license that says the license
perpetually grants me the ability to copy, modify, and distribute the work.

Hence, there might not always be a free Perl.

Is this an improper understanding?  If so, please enlighten me.


[1] These being relevant powers of copyright law for computer programs one
    commonly talks about when discussing licensing of Free Software.