What's the difference between the strings $dfull in test2 and $full in test 3?

Just to retrace your steps a bit, do you understand the difference between  $dfull in dirtest2 and  $full also in dirtest2? Had strictures (see strict) been enabled, Perl would have refused to compile  $full because it was not pre-defined. Without strictures but with warnings enabled, Perl would have told you that  $full was undefined wherever you used it in this state. These two pragmata can be enormously useful to the learning (and even the experienced) programmer. A word to the wise...

And certainly do study and understand the difference between interpolating (double-quotish) and non-interpolating (single-quotish) string operators.