sub test {
    return () ;
}
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That's because the array contains one value : 'undef' .

That looks like a string with the text "undef" to me. That's not what the array contains. Instead, it holds an undefined scalar value.

Look at what happens if you do :

sub test { return () ; }
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Lo and behold, it works!

Still, testing definedness of an array is a very bad idea. Try running this code first:

@a = qw(a b c);
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and now you'll see, the array will be defined (and empty).

Don't. Do. It.

Definedness of an array is nothing a newbie should ever be concerned about.

Still, testing definedness of an array is a very bad idea. Try running this code first:

@a = qw(a b c);
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Ah yes :) that's very true !  scalar @array is, of course, much better !

( i did meant the undef value, btw, and not the string 'undef' but it's true that my use of single quotes was indeed a bad idea !! )

I'll add something : suppose the OP wanted to undef arrays in a sub . I think I would pass the array as a reference, undef it in the sub and return the reference .

> Still, testing definedness of an array is a very bad idea.
> Try running this code first: 
>
> @a = qw(a b c);
>
> and now you'll see, the array will be defined (and empty).

Hmm, i get "defined: yes" and "size: 3" ...

@a = qw/a b c/;
printf "defined: %s\nsize: %d\n"
  , defined @a ? 'yes' : 'no'
  , scalar @a
  ;
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... what did i miss?

Do not use & unless you know what you are doing.

Running your code under strict and warnings yields a message about using defined for arrays and invites to use just @a.

use strict;
use warnings;

sub test {
  undef;
}

my @a = test;
if (@a) {
  use Data::Dumper; print Dumper \@a;
}
__END__
$VAR1 = [
          undef
        ];
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I hope this answers your question.

The Perl documentation warns you about using defined on aggregates. Usually, Perl is very DWIM, but in this case, it isn't. The return values are usually surprising, and while p5p has talked about changing this, it hasn't happened yet.

If you want to find out whether an array has elements in it, just use the array in boolean (scalar) context. It will return a true value if there are elements in it, or else it will return a false value.

Use of "defined" on aggregates (hashes and arrays) is depre-
cated.  It used to report whether memory for that aggregate has
ever been allocated.  This behavior may disappear in future
versions of Perl.  You should instead use a simple test for
size:
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It prints defined because it is having a a value undef inside it. It gave me warning that defined(@array) is depreciated ;-(