It's a bug. It works ok in 5.6.1. From perldata:

       It is often more readable to use the "=>" operator between
       key/value pairs.  The "=>" operator is mostly just a more
       visually distinctive synonym for a comma, but it also
       arranges for its left-hand operand to be interpreted as a
       string--if it's a bareword that would be a legal identi-
       fier.
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Bar::Baz is a valid identifier.

I'll perlbug it.

Abigail