Answer:

The ${*$s}{key} construct is in fact part of the *foo{THING} syntax (specifically *foo{HASH}) it just looks a little different: The first pair of braces in ${}{} expects something that evaluates to a hash reference, so when given a glob it translates that to *glob{HASH}.

Symbol::gensym() (used by the IO modules to get a reference to a new glob) removes the entry from the symbol table which makes it harder to find (like the namespace::*clean family of modules do with subs).

So these three lines are equivalent (assuming $s = \*Symbol::GEN0;) except that when using gensym() you can't use the latter two to refer to the hash by name anymore:

Thanks to BrowserUk for pointing us to the answer.

If you want some more interesting reading take a look at these gists from kentnl:

Original Question:

Where can I find the gory details of this construct used by glob/symbol-based modules (IO, Net::FTP, etc):

${*$sym}{hash_key} = 'something'

(I think) I understand the end result, but I'm interested in the how/why of it.

Where can I find documentation of that syntax? I don't know what it's called so I don't know how to search for it. I haven't found what I'm looking for in perldata, perlref, or perlmod talking about typeglobs and symbol tables.

I think I would understand what's happening if I could figure out how else to access this "hash" but I can't figure out how else to "spell" the hash that's being created. 

  DB<1> x ${*STDIN}{goo} = "ber"
0  'ber'
  DB<2> x \%{*STDIN}
0  HASH(0x9730178)
   'goo' => 'ber'
  DB<3> x \%STDIN;
0  HASH(0x9730178)
   'goo' => 'ber'

  DB<4> use Symbol

  DB<5> x $s = Symbol::gensym
0  GLOB(0x99fecc8)
   -> *Symbol::GEN0
  DB<6> x ${*$s}{pea} = 'nut'
0  'nut'
  DB<7> x \%{*$s}
0  HASH(0x9965e48)
   'pea' => 'nut'

[download]

Is there another way to access that hash? I can't find the equivalent of the STDIN example. Where is it stored? Is it a hash that's magically not accessible by name?

Thanks!

In reply to symbol/glob hash explanation by rwstauner

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