With the comma list of prints your just making a list and ignoring it's acutal value
think about this

@a=(foo,bar,bah);
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valid code, right? now how bout this?

(foo,bar,bah);
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Again valid code, it doesn't do anything but it's valid and will get evalated. Code doesn't HAVE to do something. Now if you can also do

@a=(joe(),someothersub());
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This will set @a to the return values of joe and someothersub, your doing the same thing with print

(joe(),someothersub());
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or

(print("hello"),print "\n");
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This is also valid code, calls both functions puts their return values into a list, and then forgets the list.
(feel free to correct me if I'm wrong)