First you say keys are scalars not strings, then you say there might be a way of allowing things other than strings.

Except possibly through an XS trick, keys can only be strings. perldata confirms this: "Hashes are unordered collections of scalar values indexed by their associated string key." If you try to use a reference as a key, the reference will not be a key of the hash. It's string representation will.

I guess your point was that you could attempt to use a non-string as a key. That is indeed valid. The scalar will be stringified before being used as a key.

The following is an attempt to use a reference as a key in a hash:

use strict;
use warnings;

my @var = qw( a b c );
my $ref = \@var;

my %hash;
$hash{$ref} = 1;

foreach (keys %hash) {
   print @$_, "\n";
}
[download]

In the latest Perl (5.8.8), the above code outputs

Can't use string ("ARRAY(0x225fb8)") as an ARRAY ref while "strict ref
+s" in use
[download]