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;

[download]
the above code prints 
--- 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
        };

[download]

When I undef a hash slice, all the values becomes undefined, and when I undef an array slice, only the last element becomes undefined.
Why is it so is the question?.
UPDATE
The question can be asked like this, why the undef behaves differently when it is a new array/hash, compared to when it sees an existing array/hash 
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

[download]
-- In accordance with the prarabdha of each, the One whose function it is to ordain makes each to act. What will not happen will never happen, whatever effort one may put forth. And what will happen will not fail to happen, however much one may seek to prevent it. This is certain. The part of wisdom therefore is to stay quiet.

In reply to Re^2: Hash slice again by targetsmart
in thread Hash slice again by targetsmart

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