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my %hash = ( all => 'old' );       # simple hash value
foo(\%hash);                       # pass reference to hash
...
                                   # this creates a *new* hash and a n
+ew reference in $ref
                                   # %hash is not modified
}

my %hash = ( all => 'old' );       # simple hash value
foo(\%hash);                       # pass reference to hash
...
  $ref->{ all } = 'new';           # overwrite the value to the key 'a
+ll' 
                                   # in the hash pointed to by $ref (w
+hich is %hash)
}