In your update, you are mixing up arrays and hashes. You imply this code, which you should have shown to your helpful friends use strict and use warnings:

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

@a = (a,b,c);
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undef @a{@a};# there is no hash you probably want something like this:

@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]

Be well,
rir