Why does Perl allow pass arguments by reference?

Question

I'm wondering why Perl has ability to pass argument by reference to function? I know that neither Python, nor Ruby doesn't have such feature.

Answer 1

It's useful to distinguish one thing from another.

(1) Passing arguments to a subroutine by reference. This is useful in Perl because the language passes all arguments to a subroutine as an undifferentiated list of values. Without the ability to passed data structures by reference, the designer of a function taking two lists, for example, would not be able to keep the lists separate. In addition, if the data structures are large, passing them by reference can provide a performance gain.

process_two_lists( @x,  @y); # Can't distinguish the lists.
process_two_lists(\@x, \@y); # Can.

Because Python and Ruby are designed differently, they don't require this distinction in how arguments are passed. A similar method in Python or Ruby would receive two distinct arguments (two objects representing lists x and y).

(2) Perl's behavior whereby @_ serves as an alias to the passed arguments, allowing the subroutine to modify data as perceived by the caller.

sub add_ten_to_me {
    $_[0] += 10;
}

my $x = 1;
add_ten_to_me($x);
say $x;            # 11  Amazing!

Python and Ruby can do this type of thing as well; however, there are some qualifications. Python distinguishes between mutable and immutable objects. If you pass something mutable to a Python method (a list, for example), the method is able to modify the data structure. So a Python version of process_two_lists would be able to modify both x and y. However, a function receiving immutable objects (an integer, for example) would not. Thus, a direct Python analog of add_ten_to_me would not work. [I believe that similar points could be made about Ruby, but I'm less familiar with the details at this point.]