sub rotate_logs {
  my @files = target_files_by_date_in_dir( ... );
  my $cent  = size_in_percentage( \@files );
  print "# size_in_percentage is $cent\n";
  while( $cent > 80 ){
     my @ten = splice @files, 0, 10;
 ## should be path()s already but just in case
     for my $file ( @ten ){
         print "# removing oldest $file\n";
         path( $file )->remove;
     }
     $cent = size_in_percentage( @files );
  }
  print "# size_in_percentage is $cent\n";
}
sub target_files_by_date_in_dir {
   use Path::Tiny;
   map  { $$_[0] }
   sort { $$b[1] <=> $$a[1] }
   map  { [ $_, $_->stat->mtime ] }
   map  { path( $_ )->children }
   @_ 
}
sub size_in_percentage {
   use Capture::Tiny;
   my( $files ) = @_;
   my ($du_stdout, $du_stderr, $df_exit) = capture {
     system( 'du', ..., @$files );
   };
   my ($df_stdout, $df_stderr, $df_exit) = capture {
     system( 'df', ... );
   };
   ... # math
   return $size;
}
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