use strict;

my @array = ( );

foreach (
  ['initially empty', sub {                }],
  [' undef as first', sub { @array = undef }],
  ['        emptied', sub { @array = ( )   }],
  ['        undefed', sub { undef @array   }],
  ['  emptied again', sub { @array = ( )   }],
) {
    my($name,$change) = @$_;

    $change->();

    printf "%15s:  %-5s  %-9s\n",
        $name,
      ( @array          ? 'true'     : 'false'     ),
      ( defined(@array) ? 'defined'  : 'undefined' );
}
__END__
initially empty:  false  undefined
 undef as first:  true   defined        ***********
        emptied:  false  defined    <-- ** HUH?! **
        undefed:  false  undefined      ***********
  emptied again:  false  undefined
[download]

I can reproduce the results under both 5.00503 and 5.6.1, but while I was ready to start screaming 'BUUUUUUUUGGG!', I then noticed 5.6.1 also spits out the following with warnings are enabled ...

defined(@array) is deprecated
(Maybe you should just omit the defined()?)
[download]

Something to watch out for, I guess!

    --k.