Fix multiple localized regex capture variables on both backends

[fglock]

Summary

PerlOnJava does not correctly update multiple localized regex capture variables. After local($1, $2, $3), a successful three-capture match updates $1, but $2 and $3 remain undefined on both execution backends.

This breaks MIME-tools 5.517 header, encoded-word, and parameter parsing because the distribution deliberately localizes capture variables in several parsing routines.

Minimal reproducer

use strict;
use warnings;

my $text = '=?US-ASCII?Q?Keith_Moore?=';
local($1, $2, $3);
pos($text) = 0;
$text =~ m{\G=\?([^?]*)\?([bq])\?([^?]+)\?=}xgi or die "no match";

my ($charset, $encoding, $payload) = ($1, lc($2), $3);
print "values=[$charset][$encoding][$payload]\n";

System Perl:

values=[US-ASCII][q][Keith_Moore]

PerlOnJava JVM backend:

Use of uninitialized value in concatenation (.) ...
values=[US-ASCII][][]

PerlOnJava interpreter backend produces the same incorrect values.

Removing local($1, $2, $3) makes the same regex populate all three captures on both PerlOnJava backends, so the defect is specifically tied to localization/dynamic scoping of the numbered capture variables.

CPAN integration evidence

Archived run: 20260825-173145-22349

Distribution: DSKOLL/MIME-tools-5.517.tar.gz

Requested module: MIME::Entity

PerlOnJava result:

Files=40, Tests=463
Result: FAIL
Failed 15/40 test programs. 70/463 subtests failed.

MIME-tools uses localized capture lists in:

• MIME::Words::decode_mimewords: local($1,$2,$3)
• MIME::WordDecoder: three- and five-capture variants
• MIME::Field::ParamVal: three- and five-capture variants

This directly causes encoded MIME words, charsets, filenames, and RFC 2231 parameters to become empty or undefined. A focused MIME::Words example decodes:

=?US-ASCII?Q?Keith_Moore?= <moore@example.test>

as:

 <moore@example.test>

instead of:

Keith Moore <moore@example.test>

System Perl baseline

The exact 15 test programs that fail under PerlOnJava pass under system Perl:

Files=15, Tests=242
All tests successful.
Result: PASS

The complete system-Perl suite also passes every functional MIME test. Its only failure in the restricted test environment is t/Smtpsend.t, which cannot create its local test socket. PerlOnJava skips that test because process forking is unavailable; it is unrelated to this regex defect.

Related observations outside primary scope

The archived run repeatedly warns Unknown layer: scalar, but a focused <:scalar/>:scalar read/write reproducer succeeds on system Perl and both PerlOnJava backends. Those warnings should not be treated as the cause of the localized-capture failure.

Some MIME-tools failures may remain after this fix, notably the UU-decoder size mismatch. Retest the distribution after fixing capture localization and track genuinely independent residual failures separately.

Acceptance criteria

• Add a permanent project-owned regression test for multiple localized numbered capture variables.
• Verify the test first on system Perl.
• Make the reproducer pass on both the JVM and interpreter backends.
• Ensure nested localization restores the previous capture state correctly after scope exit.
• Rerun MIME-tools 5.517 and reassess any residual failures independently.