perlcheck: add script, run in CI, fix fallouts

Add script to run all Perl sources through `perl -c` to ensure no
issues, and run this script via GHA/checksrc in CI.

Fallouts:
- fix two repeated declarations.
- move `shell_quote()` from `testutil.pm` to `pathhelp.pm`, to
  avoid circular dependency in `globalconfig.pm`.

Closes #18745

.github/workflows/checksrc.yml

@@ -82,6 +82,10 @@ jobs:
           source ~/venv/bin/activate
           scripts/cmakelint.sh
 
+      - name: 'perlcheck'
+        run: |
+          scripts/perlcheck.sh
+
       - name: 'pytype'
         run: |
           source ~/venv/bin/activate