Fatalization of changing 'use VERSION' while another 'use VERSION' is in scope

[jkeenan]

Per perldoc perldeprecation, we have three deprecations actually, fatalizations) scheduled for perl-5.44 (May 2026 release). This is one of them.

=head3 Changing C<use VERSION> while another C<use VERSION> is in scope

A C<use VERSION> declaration has many implicit effects on the surrounding
scope, such as L<strict> and L<feature> flags, or importing L<builtin>
functions. Once you have a C<use VERSION> statement in scope, another
C<use VERSION> statement with a different version is now deprecated since
Perl 5.39.8, due to the increasing complexity of swapping from one
prevailing version to another.

Category: "deprecated::subsequent_use_version"

This was originally scheduled for perl-5.46; see 351de21. That was subsequently changed to perl-5.44; see 69f23a3. AFAICT we don't yet have anyone working on this, nor have we received a pull request.

Assuming we're going forward with this ... volunteers? @Leont, @ap, @leonerd (PSC) any thoughts?

[jkeenan]

I began working on this ticket a few days ago. Work proceeded well at first but I ran into 3 failing tests. The failures in lib/feature.t are not entirely surprising, but I don't have a clue as to those in dist/Devel-SelfStubber/t/Devel-SelfStubber.t or lib/B/Deparse.t.

Test Summary Report
-------------------
../dist/Devel-SelfStubber/t/Devel-SelfStubber.t                    (Wstat: 0 Tests: 12 Failed: 1)
  Failed test:  6
../lib/feature.t                                                   (Wstat: 0 Tests: 107 Failed: 3)
  Failed tests:  63-65
../lib/B/Deparse.t                                                 (Wstat: 0 Tests: 376 Failed: 1)
  Failed test:  158

I'm not going to have that much time for Perl work over the next month, so I'm simply going to post what I've done on my GH and let anyone else pick this work up. The branch name is fatal-use-version and it can be found here.