I think he was just trying to explain for others starting out what arguments were and how they were passed to subroutines, I don't think the exact code matters all that much. Even clearer might be:

my ($bar, $baz) = ("Hello ", "World");
foo($bar, $baz); 
# ...
sub foo{        
     print "$_[0]";  #prints "Hello "
     print "$_[1]";  #prints "World"
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