I think you mean:

Assignment of a reference to a glob performs a bit of magic; the reference gets stuck in the appropriate glob slot.

That doesn't seem to work.

#!/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;
[download]

Gives

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 };
  $a;
}
[download]

If I autovivify a hash (and an array) into the glob, dump shows them there, but although the reference assignment doesn't produce errors, dumping the glob after doesn't show anything in the hash slot?

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.

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