I'm guessing you mean a data structure that might be something like:

my $ds = [
    { a => 1, b => 2 },
    [ qw{ q w e r t y } ],
    { c => 3, d => 4 },
];
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The keys of the third element can be found with: keys %{$ds->[2]}

If I'm not even close here, please provide an example of the data structure and indicate which keys you're trying to retrieve.