use Benchmark qw(timethese cmpthese countit timestr);

my $hash1 = {
  one => 'One',
  two => 'One',
  three => 'One',
  four => 'One',
  five => 'One',
};

my $hash2 = {
  one => 'One',
  four => 'One',
  five => 'One',
};

my $hash3 = {};
for (0..1000) { $hash3->{"foo_$_"} = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" }

cmpthese (-2,{
  forloop => sub {
    my $hash = {};
    foreach my $foo ($hash1, $hash2, $hash3) {
      foreach my $key (keys %$foo) {
        $hash->{$key} = $foo->{$key} if ! exists $hash->{$key};
      }
    }
  },
  forloop2 => sub {
    my $hash = {};
    foreach my $foo ($hash3, $hash2, $hash1) {
      foreach my $key (keys %$foo) {
        $hash->{$key} = $foo->{$key};
      }
    }
  },
  whileloop => sub {
    my $hash = {};
    foreach my $foo ($hash1, $hash2, $hash3) {
      while (my($key, $val) = each %$foo) {
        $hash->{$key} = $val if ! exists $hash->{$key};
      }
    }
  },
  whileloop2 => sub {
    my $hash = {};
    foreach my $foo ($hash3, $hash2, $hash1) {
      while (my($key, $val) = each %$foo) {
        $hash->{$key} = $val;
      }
    }
  },
  slice => sub {
    my $hash = {};
    foreach my $foo ($hash3, $hash2, $hash1) {
      @$hash{keys %$foo} = values %$foo;
    }
  },
  merge => sub {
    my $hash = {%$hash3, %$hash2, %$hash1};
  },

},'auto');