use strict;
use warnings;
use Data::Dumper;

my @a = qw/a b c d/;
my @b = qw/a b c d/;
my %a;
print '--- my   @a before ---',"\n", Dumper \@a;
undef @a[1,2];
print '--- my   @a after ---',"\n", Dumper \@a;

print '--- my   %a before ---',"\n", Dumper \%a;
undef @a{@b};
print '--- my   %a after ---',"\n", Dumper \%a;

##</code><code>##

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


##</code><code>##

if @a = (1,2,3), then undef @a[1,2] is proper, undefines $a[2]
just undef @b[2], is giving me @b's 0 ,1 and 2 indexes to be undef.
if %hash = qw(a b c d) then  undef @hash{qw(a c)} is proper, undefines only $hash{c}
just undef @hash1{qw(a c}} is giving me the values of the keys a and c to be undef