use inheritance to create a brand new package, and in that package, you'll name certain methods the names that I expect, and I'll call those as object methods at the appropriate places.

Wow... this is getting too deep; I think I'll go your closure way on Monday:

my @files = ( ... );
my $count;
my %data;
IterateOver::files({
  pre => sub {
    print "Starting on file $_.\n";
    $count = 0;
  },
  loop => sub {
    if (/^[^#]/ and /^(.*?)\s*=\s*(.*?)\s*(?:#.*)$/)
    {
      $data{$1} = $2;
      ++$count;
    }
  },
  post => sub {
    print "Found $count keys in $_.\n";
    print "Total unique keys so far: ", scalar keys %data, "\n";
  },
  @files
 );
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