my @to_backup = File::Find::Rule->file()
                                ->...
                                ->in('.')

for my $file_qn (@to_backup) {
   open(my $backup, "backup $file_qn |")
      or die;

   ...
}

##</code><code>##

my @to_backup = File::Find::Rule->file()
                                ->...
                                ->in('.')

for my $file_qn (@to_backup) {
   open(my $backup, '-|', 'backup', $file_qn)
      or die;

   ...
}