@values = qw( 00 11 22 33 );
use Data::Dumper;
print Dumper \@values;
# generates the output:
$VAR1 = [
          '00',
          '11',
          '22',
          '33'
        ];
[download]

$values[6] = "66";
print Dumper \@values;
# generates the output:
$VAR1 = [
          '00',
          '11',
          '22',
          '33',
          undef,
          ${\$VAR1->[4]},
          '66'
        ];
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The first four elements are unchanged. The fifth element, $values[4], is undefined. So is the sixth element, $values[5] ... Data::Dumper uses funny notation to say it has the same value as $values[4], in this case being undefined.

Some offices have a set of cubbyholes where the mail is put. This array is like one of those sets of cubbyholes. The first four cubbyholes contain pieces of paper with the text, "00", "11", "22", "33". The next two cubbyholes are empty. They don't contain pieces of paper, but the cubbyholes exist. Pieces of paper could be placed in them, in which case they would become defined. The last cubbyhole contains a value, like the first four.

Lets change things, so the every other cubbyhole has a value, and the others don't.

The fifth array slot needs to be defined:

$values[4] = "44";
 print Dumper \@values;
# generates the output
$VAR1 = [
          '00',
          '11',
          '22',
          '33',
          '44',
          undef,
          '66'
        ];
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Now there are some elements we want to get rid off. You can't assign 0, because that's a value. So is '', that's an empty string, same as "". Even assigning the value undef doesn't work!

$values[1] = undef;
print exists $values[1];
# generates the output: 1, i.e., the array slot contains the value 'un
+def'.
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The only way to make the array element have no value is to delete the value:

delete $values[1];
delete $values[3];
print Dumper \@values;
# generates the output:
$VAR1 = [
          '00',
          undef,
          '22',
          ${\$VAR1->[1]},
          '44',
          ${\$VAR1->[1]},
          '66'
        ];
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As you can see, four elements have values, the other three are undefined.This is what it means to delete elements from an array.

---

Note the subtle difference between an array element which is not defined, and an array element which is defined to contain the value undef.

$values[6] = undef
print Dumper \@values;
# generates the output :
$VAR1 = [
          '00',
          undef,
          '22',
          ${\$VAR1->[1]},
          '44',
          ${\$VAR1->[1]},
          undef
        ];
[download]

Notice that $values[6] does not use the funny Data::Dumper notation to say it is identical to elements 1, 3, and 5. That's because those three are undefined, and this one is defined. Let's continue assigning undef.

$values[0] = undef;
$values[2] = undef;
$values[4] = undef;
print Dumper \@values;
# generates the output :
$VAR1 = [
          undef,
          undef,
          undef,
          ${\$VAR1->[1]},
          undef,
          ${\$VAR1->[1]},
          undef
        ];

$values[1] = undef
$values[3] = undef
$values[5] = undef
print join( "\n", \$values[0], \$values[1], \$values[2],              
+  
                  \$values[3], \$values[4], \$values[5], 
                  \$values[6] );
# generates the output :
SCALAR(0x8279dd4)
SCALAR(0x81d64e0)
SCALAR(0x8279d50)
SCALAR(0x81d6294)
SCALAR(0x8279d38)
SCALAR(0x81d627c)
SCALAR(0x8279e10)
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See? Each array element contains a separate scalar, each with the value, undef. On the other hand, deleting each element has a different effect:

delete @values[0];
delete @values[1];
delete @values[2];
delete @values[3];
delete @values[4];
delete @values[5];
delete @values[6];
print Dumper \@values;
# generates the output :
$VAR1 = [];
[download]

Once you delete all the elements, even the array slots disappear! Of course, the more efficient way to empty the array is to assign an empty list:

@values = ();