my (%fruits,%vegetables);
   $fruit{apple}=1;
   $fruit{orange}=1;
   $fruit{banana}=1;
   $vegetable{corn}=1;
   $vegetable{carrot}=1;
   
   #test whether apple is in the set fruit
   if($fruit{apple}){  #test for inclusion in a set
      print "apple is a fruit";
   }
[download]

Using a hash slice

foreach (@fruit{'apple','banana','plum'}){$_=1}
[download]

#This will NOT work as intended
$value="'apple','banana','plum'";
foreach (@fruit{$value}){$_=1}
#doesn't work!
[download]

Often you will not know the set you want at write-time, so you will want to be able to push and pop the set from an array. Converting an array into a set can be done like:

@array=("apple","banana");
push @array, "plum";
foreach (@array){$fruit{$_}=1}
[download]

foreach (@array){$fruit{$_}=1} # initialize fruit set
[download]

  my %fruit;
  @fruit{qw/apple orange banana/} = ();
  warn "I know how to defend myself\n"
    if exists $fruit{banana};

The repeat operator (x) can be used with hash slices to get an easy to read set representation:

my @fruit_set = qw/apple banana orange mango/;
my @vegie_set = qw/onion tomato lettuce eggplant/;

my (%fruit, %vegetable);
@fruit{@fruit_set}     = (1) x @fruit_set;
@vegetable{@vegie_set} = (1) x @vegie_set;
[download]

Re: How do I use a hash as a set?

Originally posted as a Categorized Answer.