To elaborate on that second part...

When you say %hash = (a => "foo", b => "bar") perl treats the "=>" operator as a synonym for ",", so this really looks like %hash = (a,"foo",b,"bar"). Perl knows to treat a list assigned to a hash as key/value pairs, so this works right.

In your example, where you tried %hash = (a => (foo,bar, baz)), perl treats the right hand side as (a,(foo,bar,baz)), which is "flattened" into (a,foo,bar,baz), which is then assigned to %hash, yielding %hash{a} = "foo" and %hash{bar} = "baz", which isn't what you wanted.

The suggestions to use references will do what you want.