This example confuses the issue. One might conclude that the + (plus) is necessary, when in fact it's not. It's only there to workaround print's brain-damage. It could be written less ambiguously as:

  my $dayOfMonth = (localtime)[3];
  print "Today is day $dayOfMonth.";
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Note to gtrain: this is technically known as an array slice. You are taking the array of values returned by localtime and slicing out only the fourth (offset 3) element).

  my $dayOfMonth = (localtime)[3];
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In Perl terminology this is not an array slice, a slice would contrain contain more than one element . An array slice expresses a list, not a scalar. An array slice expresses a list, not an array.

Be well,
rir