If I autovivify a hash ...

That turns out to be the problem. When you obtain a new glob, it doesn't have anything in the slots and attempting to assign a reference directly into one of the empty slots silently fails to do anything.

But, if the glob already has something in a particular slot, you can overwrite it by a direct reference assignment:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dump;
use Symbol;

my $glob = Symbol::gensym;

warn "$glob\n";
dump $glob;

*{ $glob } = { a => 1 };

warn "$glob\n";
dump $glob;

%{ *$glob } = ( 'a' => 1 );
@{ *$glob } = ( 1 .. 5 );

warn "$glob\n";
dump $glob;

*{ $glob } = { b => 2 };

warn "$glob\n";
dump $glob;

__END__
C:\test>junk5
GLOB(0x225130)
do {
  require Symbol;
  Symbol::gensym();
}
GLOB(0x225130)
do {
  require Symbol;
  Symbol::gensym();
}
GLOB(0x225130)
do {
  require Symbol;
  my $a = Symbol::gensym();
  *{$a} = [1, 2, 3, 4, 5];
  *{$a} = { a => 1 }; ## Once the hash slot is in use
  $a;
}
GLOB(0x225130)
do {
  require Symbol;
  my $a = Symbol::gensym();
  *{$a} = [1, 2, 3, 4, 5];
  *{$a} = { b => 2 }; ## you can overwrite it by direct assignment.
  $a;
}
[download]

There is a bug report in there somewhere.