print needs to evaluate the function to see what is returned to be printed
Not this fragment is useful, but you can see why one would want the function to return a value first.

use strict;
use warnings;

sub yaya {
 return " ha \n";
}

my $foo = "bar";
print ($foo, yaya);
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Update:Ok. Took a look at that section of the Camel.
It's all in the precedence of arguments and functions can be arguments. parens can modify this, but you may need a unary plus to avoid some mistakes It might be worth reading that section a few more times.