>perl -le"@a=qw(a b c d e); undef @a[2,3]; print for @a"
a
b
c

e
[download]

Update: Wait, I guess it didn't work for me. (Sorry, was rushed.) You can use this:

>perl -le"@a=qw(a b c d e); undef $_ for @a[2,3]; print for @a"
a
b


e
[download]

The docs are quite specific as to what undef accepts for args. I would expect a built-in to complain if you gave it incorrect arguments, but that's what you are doing.

By the way, undef @h{@k}; is not allowed and doesn't work either.

$ perl -le"%h=map { $_=>$_ } qw(a b c d e); undef @h{qw( c d )}; print
+ qq{$_: $h{$_}} for sort keys %h"
a: a
b: b
c: c
d:
e: e
[download]

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;
[download]

--- @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
        };
[download]

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.

You are creating hash elements with undef @a{@b}. This is called autovivication (see wikipedia for a nice explanation) and has nothing to do with the undef function. Something similar happens when you write @a{@b}= (1);. The hash would contain key a set to 1 and keys b and c set to undef

@a = (a,b,c);
undef @a[1,2];

@a = (a,b,c);
[download]

@array = ( 'a', 'b', 'c', 'd', 'e' );
%hash  = ( a => 0, b => 1, c => 2, d => 3, e => 4 );
@indices = ( 2, 3);
@keys = ( 'c', 'd' );

undef @array[ @indices ];  # undefs value of/in @array[3]

undef @hash{ @keys }; # undefs value of/in $hash{d}

use Data::Dumper;

print Dumper \%hash;

print Dumper \@array;
__END__
$VAR1 = {
          'e' => 4,
          'c' => 2,
          'a' => 0,
          'b' => 1,
          'd' => undef
        };
$VAR1 = [
          'a',
          'b',
          'c',
          undef,
          'e'
        ];
[download]

He's asking why 'c' is still in the array. Even with your fix, it's still in the array.