perl_cpan_polymake_prereq: Update install instructions

[mkoeppe]

On macOS, it has gotten harder to install all Perl libraries needed for polymake.

• To clean out previous installations of Perl packages:

  rm -rf ~/perl5
  

• System /usr/bin/perl on macOS Catalina is 5.18, does not have EXTERN.h. To compile Perl packages, need to make headers from XCode available by setting CPATH. Also need to override various test failures that seem to come from system integrity protection using cpan install -f.

  env CPATH="$(xcrun --show-sdk-path)/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE:$CPATH" /usr/bin/cpan install -f XML::Writer XML::LibXML XML::LibXSLT File::Slurp JSON SVG || echo "(ignoring error)"
  

  (This setting of the environment variable should not be made persistent.)

• homebrew's perl5 .... info needed

• perlbrew (current recommendation by Perl users): https://perlbrew.pl/ https://github.com/gugod/App-perlbrew

• https://github.com/tokuhirom/plenv (alternative to perlbrew)

Depends on #31840

CC: @yuan-zhou @jplab @kliem @sophiasage @tscrim

Component: packages: optional

Issue created by migration from https://trac.sagemath.org/ticket/31830

[mkoeppe]

Dependencies: #31840