$thing will contain a hash reference, or more precisely, a reference to an anonymous hash constructed by the { ... } operator, which in this usage is a hash reference constructor.

You can refer to the hash as a reference ($thing), or as a hash (%{$thing} or %$thing), and you may refer to its individual elements by dereferencing it: $thing->{'a'}.

This would be a named hash:

my %thing = (
    a => $a,
    b => $b,
);
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