I usually do it this way:

• If I want to initialize a hash just to know whether or not a word has been seen:

  my %hash1;
  @hash1{qw/shave the modern way/}= ();
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• If it's done in a loop, when I read word one after the other, maybe from a database query or some other input

  ++$hash{$the_word_just_read};
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  Even if I don't need the actual counter, I prefer this over

  $hash{$the_word_just_read}= undef; # or 1 or ''
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To be honest: I don't really care for the efficiency of that.