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!