The ++ clearly applies scalar, lvalue context to the hash slice, and the resulting behavior shouldn't be a surprise;

sub lst:lvalue {$hash{foo},$hash{bar},$hash{baz}} (lst())++
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has a similar, but perhaps less unexpected, effect. But I don't believe the effect of putting a hash slice in scalar (much less scalar lvalue) context is anywhere defined (like so much else in perl). I wouldn't use it; I could see it triggering a warning or even an error in future perls, just as does:

($hash{foo},$hash{bar},$hash{baz})++
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