Are you sure about that? I can guarantee to you that it won't make garbage collection! First because we don't have any note about that in the POD of Symbol, and because gensym() just return a reference of a GLOB in the package Symbol::, and any GLOB reference doesn't have scope, since it's a GLOBAL "object". The best way is to see the source of gensym():

sub gensym () {
    my $name = "GEN" . $genseq++;
    my $ref = \*{$genpkg . $name};
    delete $$genpkg{$name};
    $ref;
}
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Soo, when the $ref goes out, the GLOB still exists in the package Symbol::.

Here's a test of that:

  {
    my $glob = gensym() ;
    ${*$glob} = 123 ;
    print "SCOPED: " . "${*$glob}\n" ;
  }
  print "OUT: " . ${*Symbol::GEN0} . "\n";
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SCOPED: 123
OUT: 123
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About the undef of types, well, this was already discussed here in PM, and is all about memory usage. If we make a undef in all the different types of a GLOB to clean a package from the memory, we will free much more memory than just make a undef *GLOB. And the memory difference is big. In my tests after 1000 executions I have the interpreter with 8Mb, and with the simple undef *GLOBS I have 150Mb!

  {
    my $glob = gensym() ;
    ${*$glob} = 123 ;
    print "SCOPED: " . "${*$glob}\n" ;
  }
  print "OUT: " . ${*Symbol::GEN0} . "\n";
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I don't think you quite understand what that is doing. At compile time, that *Symbol::GEN0 in the OUT line causes the GEN0 glob to be created and referred to directly by the compiled code. Then at runtime, gensym fetches and deletes it from the symbol table (skipping creating it since it's already there). Adding to your test:

  use Symbol;
  print "Symbols: ", join(",",keys %Symbol::), "\n";
  {
    my $glob = gensym() ;
    ${*$glob} = 123 ;
    print "SCOPED: " . "${*$glob}\n" ;
  }
  print "OUT: " . ${*Symbol::GEN0} . "\n";
  print "Symbols2: ", join(",",keys %Symbol::), "\n";
  print "OUT2: ", ${"Symbol::GEN0"} . "\n";
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gets you:

Symbols: qualify,geniosym,gensym,GEN0,EXPORT,EXPORT_OK,import,ungensym
+,qualify_to_ref,EXPORT_FAIL,delete_package,ISA,BEGIN,VERSION
SCOPED: 123
OUT: 123
Symbols2: qualify,geniosym,gensym,EXPORT,EXPORT_OK,import,ungensym,qua
+lify_to_ref,EXPORT_FAIL,delete_package,ISA,BEGIN,VERSION
OUT2:
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About the undef of types, I don't remember ever seeing your actual tests, and I wasn't able to deduce anything useful from your Safe::World. If undefing the individual parts of the glob is freeing more than undefing the whole glob, it is a bug and sample code showing it in action would be very welcome.

I think the symbol gets garbage collected on newer Perls, but on older ones, you must explicitly ungensym it.

Update: Yes, here's what it says:

from perldoc Symbol:

"Symbol::gensym" creates an anonymous glob and returns a reference to it. Such a glob reference can be used as a file or directory handle.

For backward compatibility with older implementations that didn't support anonymous globs, "Symbol::ungensym" is also provided. But it doesn't do anything.