or do you want to alter the stash slot holding the glob?

I think in this case you need to return a reference to a glob and dereference it with *

  DB<101> $a=42
 => 42

  DB<102> sub ret_gr { \*a }

  DB<103> $b=666
 => 666

  DB<105> *{ret_gr()}=*b

  DB<106> $a                       # *a is now an alias of *b 
 => 666

  DB<107> $b
 => 666

  DB<108> $a=0
 => 0

  DB<109> $b
 => 0
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