I did try that and it does not work, at least not on my 5.10.1

  DB<2> $s = Symbol::gensym

  DB<3> x ${*$s}{goo} = 'ber'
0  'ber'
*  DB<4> x \%{*$s}{goo}
syntax error at (eval 8)[/usr/share/perl/5.10/perl5db.pl:638] line 2, 
+near "}{goo"
*  DB<5> x \%{*$s}
0  HASH(0x86285e8)
   'goo' => 'ber'
*  DB<6> x \%{*Symbol::GEN0}
0  HASH(0x87144e0)
     empty hash
*  DB<7> x ${*Symbol::GEN0}{goo}
0  undef
  DB<8> x $]
0  5.010001
  DB<9> x $s
0  GLOB(0x86285b8)
   -> *Symbol::GEN0
[download]

...

*  DB<8> x \%{*Symbol::GEN0}
0  HASH(0x8aab260)
     empty hash
*  DB<9> x \%{*{Symbol::GEN0}}
0  HASH(0x8aab260)
     empty hash
*  DB<10> x \%{*{*Symbol::GEN0}}
0  HASH(0x8aab260)
     empty hash
*  DB<11> x \%{'' . *$s}
0  HASH(0x8aab260)
     empty hash
*  DB<12> x \%{*$s}
0  HASH(0x8a5b5e8)
   'goo' => 'ber'
*  DB<13> use Scalar::Util qw(refaddr)

  DB<14> x \%{*refaddr($s)}
syntax error at (eval 20)[/usr/share/perl/5.10/perl5db.pl:638] line 2,
+ near "*refaddr("
*  DB<15> x \%{refaddr($s)}
0  HASH(0x8b6eea0)
     empty hash
*  DB<16> x \%{sprintf "%x", refaddr($s)}
0  HASH(0x8b73ef0)
     empty hash
*  DB<17> x \%{*{"$s"}}
0  HASH(0x8b73c10)
     empty hash
[download]

It does not show up when I dump \%:: (or \%Symbol or \%Symbol:: or \%Symbol::GEN0 or \%Symbol::GEN0:: either (excluded for brevity) at least not that i can see.

Looking into Symbol, that is apparently deliberate:

my $genpkg = "Symbol::";
my $genseq = 0;

sub gensym () {
    my $name = "GEN" . $genseq++;

    ## Instantiate the glob and take a reference to it.
    my $ref = \*{$genpkg . $name}; 

    ## Delete the name, leaving a reference to an anonymous glob
    delete $$genpkg{$name};        

    ## return the ref
    $ref;
}
[download]

Thus, it effectively mirrors the behaviour [] and {} in that you have a reference to a structure that doesn't (no longer) appears in the pad (symbol table).

The few times I've wanted an anonymous glob I've used:

$r = do{ \local *FRED };;
print $r;;
GLOB(0x4055f70)
[download]

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Aha! Thank you!

I definitely should have looked at Symbol.pm before using it in my examples... Now I see why they suggest that you use it (and why the IO modules use it).

So taking into account that Symbol::gensym uses deliberate voodoo, going back to the STDIN example makes it more clear... It is in fact part of the *foo{THING} syntax (*foo{HASH} in these examples):

  DB<13> ${*STDIN}{goo} = 'ber'

  DB<14> x \%{*STDIN}
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<15> x \%STDIN
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<17> x *STDIN
0  *main::STDIN
*  DB<18> x *STDIN{HASH}
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<19> x *STDIN{IO}
0  IO::Handle=IO(0x97a5980)
[download]

And a full example demonstrating gensym's symbol table manipulation:

*  DB<6> $s = \*Symbol::GEN1
  DB<7> x $s
0  GLOB(0x9cca740)
   -> *Symbol::GEN1
  DB<8> x ${*$s}{goo} = 'ber'
0  'ber'
*  DB<9> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<10> x \%Symbol::GEN1
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<13> x *Symbol::GEN1{HASH}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<15> x *Symbol::GEN1{HASH}{pea} = 'nut'
0  'nut'
  DB<16> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
*  DB<17> delete $Symbol::{GEN1}
  DB<18> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
*  DB<19> x *Symbol::GEN1{HASH}
0  undef
  DB<20> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
[download]

So while there's definitely some black magic happening there (and with the symbol tables in general) I have a better idea of what is happening behind the curtain in this particular instance... I now see where that hash is going (or where it has been (or something like that)). Thank you very much!