use strict;
use warnings;
my %UH;
$UH{'k1'} = 1; # hash contains 1 item
$UH('k2'} = 1; # hash contains 2 items
$UH{'k1'} = 1; # at this point the hash still contains 2 items
for my $k (sort keys %UH){
  print "Hash contains key '$k'\n";
}