My son works at a company where they analyze source code and report on potential data privacy violations. For this they need to parse source code in various programming languages. He mentioned that one day they might need to support Perl too. I thought about the difficulties in parsing Perl and one case came to my mind was the strange implicit return from a function that Perl has.

Then I also thought that Rust also has some strange ideas. BTW Do you know that I have a new web site called Rust Maven where I write about Rust? Now you know.

Anyway, back to Perl:

What do you think this function will return and what will this code print?

examples/implicit_return.pl

use strict;
use warnings;
use feature 'say';


sub function {
    my ($x, $y) = @_;

    if ($x < $y) {
        my $total = $x + $y;
    }
}

{
    my $res = function(2, 3);
    say defined $res ? "defined" : "undef";
    say $res eq "" ? "empty string" : "NOT the empty string";
    say $res;
    say "-------------------";
}
{
    my $res = function(3, 2);
    say defined $res;
    say $res eq "" ? "empty string" : "NOT the empty string";
    say $res;
    say "-------------------";
}

I know it is a very contrived example, but in the few minutes I spent on it I could not come up with a more realistic one. Send me a better example!

Spoiler Alert

If a function in Perl does not explicitly call return then it will implicitly return the result of the last statement evaluated.

In the above code, if the if condition inside the "function" evaluates to true then the code inside the block will be executed and thus the assignment to $total will be the last executed statement and so the value of $total will be returned.

On the other hand if the if condition inside the "function" evaluates to false then this will be the last statement in the function and thus this false value will be returned that will happen to be the empty string.

In the example I added a check to show whether the returned values is defined or if it is undef. A separate check is included to show if it equals to the empty string or not using the ternary operator. 

defined
NOT the empty string
5
-------------------
1
empty string

-------------------