@a = qw(a b c d );
undef $a[2];

$b{x} = 10 ;
$b{$_} = 1 foreach grep defined, @a;

##</code><code>##

@a = qw(a b c d );
splice(@a, 2, 1);

$b{x} = 10 ;
$b{$_} = 1 foreach @a;