I think you'll find the enlightenment you're after in perlreftut - Mark's very short tutorial about references.

When you called $rows = $dbh->selectall_arrayref, DBI returned a single scalar value which you assigned to $rows. That value was a reference to an array. Each element in that array was a reference to a hash of column names and values.

The array-dereferencing syntax you quoted:

is exactly equivalent to this syntax:

And in your case would return a hashref.

  my $hash_ref = $array_ref->[2];
  my $value = $hashref->{$key};
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You can also chain together the dereferencing like this:

  my $value = $array_ref->[2]->{$key};
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And you can even omit the arrow when the initial data structure contains references:

  my $value = $array_ref->[2]{$key};
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