In question 1, the thing that's left around is the results of doing a stat of the file.  An implicit stat is done when you use "-s" on a file, but a stat gives you a number of other pieces of information, which you already have in-memory.

For question 2, yes, your understanding is correct.

For question 3, you are misusing "_" as you speculated.  You can't print it out, you can only use it in conjunction with one of the -x tests.  However, you could do the following to print the file when it matches the tests -s and -A:

#!/perl/bin/perl

use strict;

my @original_files = qw/ fred /;
my @big_old_files;
foreach (@original_files) {
   if ((-s) > 0 and -A _ > 0) {
      push @big_old_files, $_;
      print "$_\n";    # Display the file name if it passes -s and -A 
+tests.
   }
}
[download]

Note, too, that when you say "(-s)", you're implicitly saying "(-s $_)".