bug#31216: commit 217b8c2e0 produces 'Can't locate Date/Parse.pm in @INC Error' perl error

[Mark H Weaver]

Unfortunately, the compatibility symlinks are not sufficient.  The
problem is that other perl packages such a po4a are installing their
modules into $out/lib/perl5/site_perl/5.26.1, which conflicts with the
compatibility symlink that we're installing in perl.

When perl and po4a are put together into a profile, po4a has a directory
<PO4A>/lib/perl5/site_perl/5.26.1 and perl has its compatibility symlink
in the same relative location.  The (guix build union) code follows
perl's symlink to its target directory <PERL>/lib/perl5/site_perl/5.26.2
and merges that with <PO4A>/lib/perl5/site_perl/5.26.1 to form
<PROFILE>/lib/perl5/site_perl/5.26.1, but that not in any of the paths.

I'm now trying a different approach: hack the replacement perl to think
it is version 5.26.1, although it is actually 5.26.2.  I'll report back
soon.

       Mark