First off, "undefined array" is not a useful concept (in Perl anyway). In Perl an array can be empty or not, but not undefined. However, remember that a Perl array is an array of scalar elements and a scalar may have the value 'undef'.

Using @array in a scalar context (in a conditional statement for example) returns a number which is the number of elements in the array. Note however that that is a number so the numeric comparison operators (==, != <, ...) are appropriate, not the string comparison operators (eq, ne, lt, ...).

Now, to address the warning in your code. Consider:

use strict;
use warnings;

my @array;

print "\@array contains ", scalar @array, " elements\n";
print for @array;

@array = undef;
print "\@array now contains ", scalar @array, " element\n";
for my $index (0 .. $#array) {
    print "Element at $index: ", (defined $_ ? "'$_'" : "undefined"), 
+"\n"
}

@array = ();
print "\@array contains ", scalar @array, " elements\n";
[download]

Prints:

@array contains 0 elements
@array now contains 1 element
Element at 0: undefined
@array contains 0 elements
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Do you see that the assignment @array = undef has almost exactly the opposite effect to the one I suspect you expected? It populates the array with one element which has the value undef! If you want to empty an array assign the empty list () to it.

my @array = undef; assigns a 1-element list with value undef

HTH!

PS: writing my @a is already sufficient for having an empty array!