hippo, thanks! I had some time this morning, and started improving the code based on your comment. While there's still a lot of work to do, I added a method:

sub write_list_to_logfile {
  my ( $list, $logfile ) = @_;
  open my $file, '>', $logfile or die "Can't open $logfile: $!";
  foreach my $line ( @$list ){
    print $file "$line\n";
  }
  close $file;
}
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And then changed the logging method to use the new method:

if ( keys(%known_dirs) ) {
  my @known_dirs = keys(%known_dirs);
  my $known_dirs_filename = "$dir/known_dirs.list";
  write_list_to_logfile(\@known_dirs, $known_dirs_filename);
}
[download]

The new method works, but I ran out of time before I could streamline it more. I'm thinking I can eliminate the two assignments, but need to test the idea.