perl: pin cf_time/cf_by for reproducible builds

[bryan-minimal]

Problem

perl's Configure bakes the wall-clock build time (and builder name) into
Config_heavy.pl, perlbug, and perlthanks via cf_time/cf_by. Two builds
of the same source therefore produce different bytes. Confirmed with a
build-twice-and-diff:

build A:  cf_time='Mon Jun 15 18:48:21 UTC 2026'
build B:  cf_time='Mon Jun 15 18:50:59 UTC 2026'

Fix

Pin both values deterministically from SOURCE_DATE_EPOCH and pass them explicitly
to Configure (perl does not honor SOURCE_DATE_EPOCH natively):

export SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-0}"
CF_TIME="$(LC_ALL=C TZ=UTC date -u -d "@$SOURCE_DATE_EPOCH" 2>/dev/null || echo 'Thu Jan  1 00:00:00 UTC 1970')"
# ... Configure ... -D cf_time="$CF_TIME" -D cf_by=builder

Verification

A from-source double-build (~34 min) now produces byte-identical content —
repro-check diff reports no content differences (previously: cf_time digits
differed in the 3 files above).

Note: on a case-insensitive filesystem (e.g. macOS APFS), perl's Unicode property
tables (_PerlAny.pl, etc.) show a spurious file-set diff because their names
case-collide. That's a filesystem artifact of the test host, not a build issue —
on a case-sensitive Linux builder the output is fully reproducible.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Enhanced build reproducibility by standardizing build-time configuration settings to ensure consistent builds across different environments.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 6e6f29b and dfa3ca7.

📒 Files selected for processing (1)

• packages/perl/build.sh

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📝 Walkthrough

Walkthrough

packages/perl/build.sh gains two reproducibility fixes: it defaults SOURCE_DATE_EPOCH to 0, derives CF_TIME from it as a UTC date string (with a fallback), and forwards both -D cf_time="$CF_TIME" and -D cf_by=builder to Perl's Configure invocation.

Changes

Perl Reproducible Build

Layer / File(s)	Summary
Deterministic timestamp derivation and Configure wiring packages/perl/build.sh	SOURCE_DATE_EPOCH is defaulted to 0; CF_TIME is computed from it via date -u with a fallback literal string. The Configure call is updated to pass -D cf_time="$CF_TIME" and -D cf_by=builder.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

Poem

🐇 A timestamp once floated, unknown and free,
Now anchored to epoch, as stable as me.
cf_by=builder, no username in sight,
The bunny ensures every build is just right.
Reproducible hops, from zero to done! 🕐

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely summarizes the main change: pinning cf_time and cf_by configuration variables in the perl build script for reproducible builds.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bryan/perl-reproducible-cf-time

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