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From: PerlFAQ Server on 13 Jun 2010 00:00
This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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4.70: How can I use a reference as a hash key?

(contributed by brian d foy and Ben Morrow)

Hash keys are strings, so you can't really use a reference as the key.
When you try to do that, perl turns the reference into its stringified
form (for instance, "HASH(0xDEADBEEF)"). From there you can't get back
the reference from the stringified form, at least without doing some
extra work on your own.

Remember that the entry in the hash will still be there even if the
referenced variable goes out of scope, and that it is entirely possible
for Perl to subsequently allocate a different variable at the same
address. This will mean a new variable might accidentally be associated
with the value for an old.

If you have Perl 5.10 or later, and you just want to store a value
against the reference for lookup later, you can use the core
Hash::Util::Fieldhash module. This will also handle renaming the keys if
you use multiple threads (which causes all variables to be reallocated
at new addresses, changing their stringification), and
garbage-collecting the entries when the referenced variable goes out of
scope.

If you actually need to be able to get a real reference back from each
hash entry, you can use the Tie::RefHash module, which does the required
work for you.



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