utils/cpan: fail closed on non-CPAN and livecheck resources

[p-linnane]

CPAN.update_perl_resources! replaces the entire resource stanza group with the blocks it generates for MetaCPAN resources, so anything else living in that group is deleted. That means non-CPAN resources, and livecheck blocks inside CPAN resources, were silently discarded by an in-place update.

Eight homebrew-core formulae are affected. git loses its html and man resources, texlive makes 45 MetaCPAN requests and then drops 4 non-CPAN resources, and libbi drops 1. argus-clients, amtterm, help2man, exim, and lanraragi each lose the livecheck block on a CPAN resource.

To reproduce:

brew update-perl-resources git
git -C "$(brew --repository homebrew/core)" diff Formula/g/git.rb
git -C "$(brew --repository homebrew/core)" checkout Formula/g/git.rb

The diff shows the html and man resource blocks removed.

This adds two guards that fail closed before any MetaCPAN request or disk write, following the non-PyPI resource guard in utils/pypi.rb. --print-only is unaffected and still prints the CPAN blocks it would generate. Note this is a user-visible change: the command now refuses on those eight formulae, where it previously succeeded destructively. Teaching it to skip and preserve those resources the way utils/pypi.rb does is a follow-up; refusing is the safe intermediate state.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Opus 5) drafted the implementation and tests; I reviewed the diff, verified each new test fails without the guards and passes with them, and ran brew lgtm plus targeted specs.

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[Copilot]

Pull request overview

This pull request updates Homebrew’s CPAN resource updater (brew update-perl-resources) to fail closed when a formula contains (a) any non-MetaCPAN resources or (b) CPAN resources that define livecheck blocks, preventing destructive in-place rewrites that would silently drop those stanzas.

Changes:

• Add preflight guards in CPAN.update_perl_resources! that abort (unless --print-only) when non-CPAN resources or resource livecheck blocks are present.
• Remove the prior “skip non-CPAN resources” behaviour that could still lead to those resources being deleted from the file during rewrite.
• Add/extend RSpec coverage to ensure the command exits before any MetaCPAN request or file modification in the guarded cases, while keeping --print-only behaviour intact.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/utils/cpan.rb	Adds guardrails to abort updates when the formula contains non-CPAN resources or CPAN resource livecheck blocks (except in --print-only mode).
Library/Homebrew/test/utils/cpan_spec.rb	Adds regression tests covering failure-without-write for guarded cases and verifies --print-only still prints updated CPAN blocks.

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[MikeMcQuaid]

Nice, thanks!