This is documented, and used most often in hashes, like this:

%hash = ( foo => 'bar', # note 'foo' not needed
          bar => 'baz',
          baz => 'foo',
          items => 3 );
print $hash{items};     # note 'items' not needed
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The => acts like a special sort of comma, which quotes barewords on the left; barewords are also quoted in hash lookups.