perlcheck: parallelize

Follow-up to 34b1e146e4 #18745

Closes #18750

scripts/perlcheck.sh

@@ -32,6 +32,10 @@ set -eu
 
 cd "$(dirname "$0")"/..
 
+procs=6
+command -v nproc >/dev/null && procs="$(nproc)"
+echo "parallel: ${procs}"
+
 {
   if [ -n "${1:-}" ]; then
     for A in "$@"; do printf "%s\n" "$A"; done
@@ -44,4 +48,4 @@ cd "$(dirname "$0")"/..
     # strip off the leading ./ to make the grep regexes work properly
     find . -type f \( -name '*.pl' -o -name '*.pm' \) | sed 's@^\./@@'
   fi
-} | xargs -n 1 perl -c -Itests
+} | xargs -n 1 -P "${procs}" perl -c -Itests