From @tbrowder

According to S26, headings may be numbered in nested format​:

Alternatively, as a short-hand, if the first whitespace-delimited word
in a heading consists of a single literal C<#> character, the C<#> is
removed and the heading is treated as if it had a C<​:numbered> option​:

=begin code
  =head1 # The Problem
  =head1 # The Solution
  =head2 # Analysis
  =head3 Overview
  =head3 Details
  =head2 # Design
  =head1 # The Implementation
=end code