defined is not meant to be used on an array. For starters, it would be impossible for an array to be undefined since undef is a scalar value.

As such, it appears that defined imposes a scalar context on its argument. An array name in scalar context is the number of elements in that array. Numbers are defined, so defined(@bar) always returns true.

Furthermore, my @bar = undef; doesn't create an empty array. It creates an array with one element in it (undef). my @bar = (); and my @bar; are equivalent and create empty arrays.

If you want to check if the array is empty, just use the array name in scalar context: 

my @bar = undef;
print("There are ", scalar(@bar), " elements in \@bar\n");
print("\@bar is empty\n")    if !@bar;
print("\@bar isn't empty\n") if @bar;

my @foo;
print("There are ", scalar(@foo), " elements in \@foo\n");
print("\@foo is empty\n")    if !@foo;
print("\@foo isn't empty\n") if @foo;

[download]
There are 1 elements in @bar
@bar isn't empty
There are 0 elements in @foo
@foo is empty

[download]

You can check if an *array element* is defined, since array elements are scalars. 

my @bar = (undef, 'a');
print(defined($bar[0])?1:0, "\n");  # 0
print(defined($bar[1])?1:0, "\n");  # 1
print(defined($bar[2])?1:0, "\n");  # 0 (doesn't exist)

[download]

Update: Clarified some details
Update: "Deleted" a paragraph in response to ysth's reply.

In reply to Re: undef==defined sometimes? (body question) by ikegami
in thread undef==defined sometimes? by perl-diddler

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