There's no real question in your post, so I'll just comment on it :-)

I don't see any real problems with it (other than the fact that I prefer to group function definitions and not mix them with the rest of the code). The my (%HASH) has the effect of localizing the %HASH variable to the block where the doStuff function is declared, effectively making it invisible to everyone else and accessible only through that function. In this sense it is a good thing to have the my statement. If you remove it, the %HASH variable will be accessible from other parts in the program, which may or may not be what you want.

You mention %thisDataset a couple of times, but there is no such variable in your program. You have %HASH, and you have $thisDataset, which is a pointer to %HASH.

The use of %{$_[0]} in the second function looks good to me, but the @{$_{$KEY}} does not. I think it should be @{$_[0]->{$KEY}}. It may make things clearer to assign $_[0] to another variable, like this:

sub someThingElse {
  my $data=shift;
  # then you can use %{$data} and @{$data->{$KEY}}
}
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--ZZamboni