Your statement

 my %h = %$ptr;
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makes a (shallow) COPY of the hash, via the hash-ref.

Then

 my @a = $h{'stack'};
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make a COPY of the array-ref into the zero'th element of @a (probably not what you intended).

zwon(++)'s solution works directly off the references to the original objects.

If you want to use the syntax you are attempting, you need to make symbolic references, instead of copies:

  local *h=$ptr;  # %h now is synonymous with %debug
  local *a=\$h{stack};  # @a is another name for the anon array @{ $h{
+stack} }
  # Now, a 'push' into @a will change %debug
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(Untested). See 'Typeglobs and Filehandles' in 'Perldata'.

You will need to tweak 'use strict', to enable typeglobs.