I've investigated the merge of hashs (associative arrays) in perl. Then I've recorded it as a note for myself.

Probably the following code is the most intuitive.

%hash = (%hash, %addition);

To add %addition into %hash.

my %hash = (
  'key1' => 'value1',
  'key2' => 'value2',
);
my %addition = (
  'key3' => 'value3',
  'key4' => 'value4',
  'key1' => 'override1',
);
%hash = (%hash, %addition);

When the above code is run, %hash will be as follows:

  'key1' => 'override1',
  'key2' => 'value2',
  'key3' => 'value3',
  'key4' => 'value4',

A value of key1 is overwritten.

We can use while or foreach loop to merge hashes.

The following code uses while and each:

while (my($key,$val) = each (%addition))
{
  $hash{$key} = $val;
}

The following code uses foreach and keys:

foreach my $key (keys %addition)
{
  $hash{$key} = $addition{$key};
}

We can also use map to merge hashes such as:

map { $hash{$_} = $addition{$_} } keys %addition;

At last, we can use slice. Using "slice" is like a hack, I'm not familiar with this way.

@hash { keys %addition } = values %addition;

We should use @hash instead %hash to use slice.

Using hash reference, we can code as follows:

# normal merge
$hash = {%{$hash}, %addition};
# use while
while (my($key,$val) = each (%addition))
{
  $hash->{$key} = $val;
}
# use foreach
foreach my $key (keys %addition)
{
  $hash->{$key} = $addition{$key};
}
# use map
map { $hash->{$_} = $addition{$_} } keys %addition;
# use slice
@{$hash} { keys %addition } = values %addition;

I've taken benchmarks for those ways. The code for the benchmarks is in sequel part of this article.