Perl Build Input and PERL5LIB

1st chunk of `doc/hooks/perl.section.md`

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# Perl {#setup-hook-perl}

Adds the `lib/site_perl` subdirectory of each build input to the `PERL5LIB` environment variable. For instance, if `buildInputs` contains Perl, then the `lib/site_perl` subdirectory of each input is added to the `PERL5LIB` environment variable.

Title: Perl Build Input and PERL5LIB

Summary

This section describes how the Perl setup hook modifies the `PERL5LIB` environment variable. Specifically, it adds the `lib/site_perl` subdirectory of every Perl build input (e.g., if 'Perl' is in `buildInputs`) to `PERL5LIB`.