Just a slight expansion... The first one translates roughly to:

{   local $_;  *_= \$guarantee;
    s/this/that/g;
    s/foo/bar/g;
}
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which means that during that block you can take and keep a reference to \$_ and you'll get a reference to $guarantee (not that such happens often, but it is one way to distinguish between the two). (:

        - tye (but my friends call me "Tye")