It seems that you must have $my_hash as a hashref, rather than having a %my_hash. The arrow-form adds a dereference to the LHS value. In my example, #1 and #2 are equivalent:

my $ref = { foo => "bar" };

print ${$ref}{"foo"};  #1  prints "bar"
print $ref->{"foo"};   #2  prints "bar"
print $ref{"foo"};     #3  prints ""
      # and under strictures throws a: 
      # "Global symbol "%ref" requires explicit package name"

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$ref{"foo"} only returns a value if there is a hash named %ref.

In reply to Re^3: Getting Getopt::Long values into a hash by fishbot_v2
in thread Getting Getopt::Long values into a hash by jeanluca

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