If you want to understand what's going on, you can try adding some print statements. This should make it pretty clear what's happening.

  use strict;

  my @list = qw(1 2 3 4 5);
  my $each = "";
  my $i = 0;
  my @newlist = ();

  foreach $each (@list) {
        print "Each was: $each\n"; #new
        $each *= 2;
        print "Each is: $each\n"; #new
        $newlist[$i] = $each;
        print "New list is: $newlist[$i]\n"; #new
        print "Old list is $list[$i]\n\n"; #new
        ++$i;
  }

  for ($i= 0; $i<5; ++$i) {
        print ($newlist[$i] - $list[$i], "\n");
  }
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And maybe in the future it would be a good idea for the reason for what happens to be included in the 'black box of enlightenment' so that people won't just be puzzled, but will learn, too? (Or maybe not; it's good debugging practice to figure out where the unexpected happens. Why have the box at all? There isn't much point to just show the output, since people can just run the program to see the output.)