#push @dfdata_out, (\%dfdata_row);
    %dfdata_row = ();
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What happens here is that you push into @dfdata_out a reference to the hash %dfdata_row, and then you empty the hash. After that, the reference in @dfdata_out is the same reference pointing to an empty hash. What you probably want to do is:

push @dfdata_out, { %dfdata_row };
%dfdata_row = ();
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This is effectively the same as what's working for you now. It copies the old hash to a new (anonymous) hash and pushes a reference to the copy on the list.

You might want to have a look at perlreftut for more.