Regarding your first question, yes, that was a typo (now fixed).

A hash slice is a way to access several values of a hash using a list of keys. For regular hashes, for example,

@hash{ qw( foo bar baz ) } = ( 1, 2, 3 );
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has the same effect as

$hash{ foo } = 1;
$hash{ bar } = 2;
$hash{ baz } = 3;
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You can use slices to copy several values from one hash to another. E.g.

@keys = qw( foo bar baz );
@copy{ @keys } = @orig{ @keys };
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The last line copies three key-value pairs from %orig to %copy. In the code I posted earlier, the only difference is that the accessing of the RHS slice is done via a reference ($d) to a hash instead of a regular hash:

@h{ @flds } = @{ $d }{ @flds };  # hash slice assignment
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Once you dereference a hash ref you can use it like any other hash.

Slices are discussed in perldata, and using references in perlreftut and perlref.