The first way is straightforward, and probably desirable under many circumstances. The second way is kind of inefficient. In the second method, you're creating an anonymous hash, referred to by a temporary hashref. That hashref is then dereferenced as a hash, and assigned to %h2.

The second way takes an extra step. In the narrow context you've displayed it's completely unneeded. It's less memory and computationally efficient, and arguably less clear to the reader.

However, that's not to say there arent times when an anonymous hashref is useful. Consider the following:

my $href = { FOO => 1, BAR => 2 };

my_sub( $href );

sub my_sub {
    my $href = shift;
    # do something with the hash ref...
}
[download]

In other words, references and anonymous hashes are, theselves, useful. But the way you used them in your second example isn't.

Relevant documentation: perlref, perlreftut, and perldsc.

The second varient is stupid.

1. It builds one (anonymous) hash;
2. flattens it to a list by dereferencing it in a list context;
3. assigns the list to build a second named hash;
4. and discards the anonymous hash.

The first only does step 2.

Bah ... part of my problem was that I had tried replacing the anonymous hashref with a named hashref, and found it didn't work. Apparently, I had made some simple coding error, because the following does, in fact, work as expected:

use warnings;
use strict;

my $h = {FOO => 1, BAR => 2};
my %hash = %{$h};
print $hash{FOO}, "\n";
print $hash{BAR}, "\n";
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Incidentally, that "stupid" way of assigning hashes was taken from the Makefile.PL that Inline::C autogenerates. (I guess that has something to do with the way that Inline::C determines its parameters.)

Thanks guys.

Cheers,
Rob

Unless there is something that you've omitted by the simplification for the OP example code, there is no utility at all in building a named hash from a constant list via an anonymous hash.

In your modified example above, there is the potential for utility, though it is not realised. That of retaining a copy of the original (anonymous) hash. This only comes about because a reference to the anonymous hash is retained which it isn't in the OP.

This allows modifications to be made to the copy whilst retaining the original. This can be used for setting defaults in object constructors and the like:

package someclass;

my $defaults = { foo=>1, bar=>23 };

sub new {
    my $class = shift;
    my %self = ( %{ $defaults }, @_ );
    return bless \$self, $class;
}
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Here keys foo and bar in $self will get the default values unless they are overidden by named arguments on the constructor, and $defaults remains unmodified.

Though there is little merit in using a named anonymous hash rather than real hash unless $defaults is passed to a subroutine somewhere.

ps. I didn't say it didn't work. Only that there was no point to it! ;)

---

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