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

--- my   @a before ---
$VAR1 = [
...
          'd' => undef
        };

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