An alternative usage for what you posted, having the same end-result is 

@saw{@in} = undef;

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Because of autovivification-related bugs, I got into the habit of using defined instead of exists to determine whether a hash contains a certain key. This habit required developing the second habit of using something like 

@saw{@in} = (1) x @in;

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instead of 
@saw{@in} = ();

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As I say, these are just programming habits; with a modicum of vigilance it is perfectly possible to avoid autovivification bugs and still use the @saw{@in} = () idiom. In fact, vigilance would be cheaper than my habit, which costs me a 10% premium in the size of the resulting hashes: 
  DB<1> use Devel::Size 'total_size'
  DB<2> @x{1..1000}=()  
  DB<3> @y{1..1000}=(1) x 1000
  DB<4> p total_size(\%x)
41049
  DB<5> p total_size(\%y)
45049

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I'm willing to pay this price to avoid heart-stopping encounters with lines like this in my code 
if ( exists $some_hash{ foo } ) {
  launch_the_missiles();
}

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and the resulting frantic code scan to ensure that it doesn't contain anything like 
$x = $some_hash{ maybe_foo() }{ bar };

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Now I use exists only when dealing with the rare hash for which undef is a legitimate value.

Update: A related idiom, which, in addition to removing duplicates, also preserves the original order as much as possible, is: 

my @no_repeats = do { my %h; grep !$h{$_}++, @has_repeats };

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the lowliest monk

In reply to Re: need explanation of @foo{@bar} = (); (hash slice) by tlm
in thread need explanation of @foo{@bar} = (); (hash slice) by Anonymous Monk

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