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: Does the "dual license" arrangement allow that?

Yes. At least this is my understanding.

The Perl dual license is one part GPL - this forces free release in case of modified versions.
The other part is Artistic license.
Read http://dev.perl.org/licenses/artistic.html please. See sections 3 +4 part d)
This would allow *other distribution arrangements with the copyright holder(s)*.
My understand is that if you release this dual license *the same terms as perl itself* people will be free to pick their according license and ask the copyright holders of code they would derive from about *other distribution arrangements*.

But: this applies to *all* copyright holders someone derives a product from.
And: everybody may pick *one* of the licenses or releas a derivate on dual license again. Only those derivates linking to other GPL-only code must stick to GPL part of your license.

Example:
If you release 5.1x dual license. Tarty may pick the Artistic part, make an distribution arrangement according to part 3.d) or 4.d) of Artistic license and release his moded stuff as shareware. No problem here.
At the same time others may take your version 5.1x and develop it to whatever other product GPL or Artistic or dual license.

In other words:
Your decision does not necessarily affect Tarty's version. He may release his mods under the same terms or he may not.

I agree with what you said:
Your goal is to keep the scripts maintained and to keep your customers supported in future.

cu
Rainer