You probably mean undefined elements, NULL is a c-thing. One way to create an array with undefined elements is to use indexes and then skip a few. With grep -ing for defined elements, you can weed the undefined elements, as shown in the example below.

As mentioned, an empty array is not the same as an undefined array, and "0" is defined, so you don't want to skip it.

#!/usr/bin/perl
my (@ar1, @ar2);

$ar1[0] = 1;
$ar1[1] = 0; # a defined element, not NULL
$ar1[7] = 9; # skip, leaving 2..6 undefined

# dump array 1
print("\@ar1:\n");
for my $i (0..$#ar1) {
   print("   $i : $ar1[$i]\n");
}

# weed the undefined elements
@ar2 = grep { defined $_ } @ar1;

print("\@ar2:\n");
for my $i (0..$#ar2) {
   print("   $i : $ar2[$i]\n");
}
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