if (@a) { # @a is not empty...
  ...
} else { # @a is empty
  ...
}
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The answer here does depend a little bit on what you mean by 'empty'. The earlier answers only tell you if an array has any elements, but not tell you what they contain.

If by empty you mean 'only contains defined elements' then you will need the following test instead:

my @a = (0,1,2,3);

foreach (@a) {print "'$_'\n";} # Prints each element.

print "\@a exists\n" if @a;

print "\@a is not Empty\n" if grep {defined($_)} @a;
    # Prints '@a is not empty' if any element in '@a' is defined.

my @b = (1, undef)
foreach (@b) {print "'$_'\n";}

print "\@b exists\n" if @b;

print "@b is not Empty\n" if grep {defined($_)} @b;
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Of course, if you want to know if the array contains only true values, then you can get away without using the defined() bit.

Edited: Examples reworked by davido.

my $j = 0;

for (my $i = 0; $i <= $#array; ++$i) {
  $current = $array[$i];
  if($current == undef) {
    push @empty, $j++;
  }
}

print "undefined indices:  @empty \n";
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my @empties = ();
for my $i(0..$#array) {
   unless(defined $array[$i]) {
       push @empties, $i;
   }
}
print "undefined indices @empties\n";
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{Editor's note: This example doesn't work as the poster thought it would. See Abigail-II's explanation here: Re: Answer: How do I test if an array is empty or not?.}

Edited by davido for clarification.

What your code is trying to do is extracting the undefined values (or rather, the indices) from the array - which is a different question. However, your first code fragment not only finds the indices with undefined values, but anything that is 0 in numeric context, which includes 0, the empty string and the undefined value.

Your second code fragment is correct, but it can be written in a more Perlesque way:

    my @empties = grep {!defined $array [$i]} 0 .. $#array;
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As for the question, just use the array in scalar context. It's empty if the array is 0 in scalar context.

Abigail