use Data::Dumper;

my @a = qw/a b c d e/;
my %a;
@a{@a} = (1) x @a;

print "--- \@a before ---\n", Dumper \@a;
print "--- \%a before ---\n", Dumper \%a;

undef @a{@a};
undef @a[1,2,3];

print "--- \@a after ---\n", Dumper \@a;
print "--- \%a after ---\n", Dumper \%a;

##</code><code>##

--- @a before ---
$VAR1 = [
          'a',
          'b',
          'c',
          'd',
          'e'
        ];
--- %a before ---
$VAR1 = {
          'e' => 1,
          'c' => 1,
          'a' => 1,
          'b' => 1,
          'd' => 1
        };
--- @a after ---
$VAR1 = [
          'a',
          'b',
          'c',
          undef,
          'e'
        ];
--- %a after ---
$VAR1 = {
          'e' => undef,
          'c' => 1,
          'a' => 1,
          'b' => 1,
          'd' => 1
        };