use warnings;
use strict;

my $empty_str = q{};
my $empty_hash_ref = { q{} => q{}, };

for ( keys %$empty_hash_ref ) {
    $empty_str eq $_ ? print "PASS" : print "FAIL";  ## print PASS
}

## use smartmarch ~~
$empty_str ~~ $empty_hash_ref ? print "PASS" : print "FAIL"; ## print PASS

##</code><code>##

  my $empty_str = q{};
  my $empty_hash_ref = {};  ## why?

   for (keys %$empty_hash_ref){
       $empty_str eq $_? print "PASS": print "FAIL"; # print nothing
   }

   $empty_str~~$empty_hash_ref?print "PASS": print "FAIL"; # print FAIL