Programming Perl, 4th Edition

References Don’t Work As Hash Keys

Hash keys are stored internally as strings.^[129] If you try to store a reference as a key in a hash, the key value will be converted into a string:

$x{ \$a } = $a;
($key, $value) = each %x;
print $$key;                # WRONG

We mentioned earlier that you can’t convert a string back to a hard reference. So if you try to dereference $key, which contains a mere string, it won’t return a hard dereference, but rather a symbolic dereference—and since you probably don’t have a variable named SCALAR(0x1fc0e), you won’t accomplish what you’re attempting. You might want to do something more like:

$r = \@a;
$x{ $r } = $r;

Then at least you can use the hash value, which will be a hard reference, instead of the key, which won’t.

Although you can’t store a reference as a key, if (as in the earlier example) you use a hard reference in a string context, it is guaranteed to produce a unique string. This is because the address of the reference is included as part of the resulting string. So you can in fact use a reference as a unique hash key; you just can’t dereference it later.

There is one special kind of hash in which you are able to use references as keys. Through the magic^[130] of the Tie::RefHash module bundled with Perl, the thing we just said you couldn’t do, you can do:

use Tie::RefHash;
tie my %h, "Tie::RefHash";
%h = (
    ["this", "here"]   => "at home",
    ["that", "there"]  => "elsewhere",
);
while ( my($keyref, $value) = each %h ) {
    say "@$keyref is $value";
}

In fact, by tying ...