update: inadvertently answered monsoon instead of OP. My mistake. I read the doc quotation as the element about which OP had "doubt."</update>

Perhaps the example will help:

#!/usr/bin/perl
use 5.014;

# 980189

sub add_with_return {
    my $in_in_sub; my $in2_in_sub;
    $in_in_sub = $_[0];              # For utter clarity; no doubts; 
                                     # @_ is the array passed to the s
+ub
    $in2_in_sub = $_[1];             # $_[0] & $_[1] are the first two
+ elements of @_
    my $out_in_sub = $in_in_sub + $in2_in_sub; 
    return $out_in_sub;
}

sub concat_w_NO_return {
    my ($in_in_sub, $in2_in_sub);
    $in_in_sub = $_[0];              # For utter clarity; no doubts; 
                                     # but MANY simpler & BETTER ways 
+to do this
    my $in2_in_sub = $_[1];
    my $concat = "$in_in_sub" . "$in2_in_sub";  # Last statement, no r
+eturn; so the
}                                               # computed value of $c
+oncat gets returned

my $in = 1;
my $in2 = 2;
my $str1 = "foo";
my $str2 = "bar";

my $sum = add_with_return($in, $in2);
say "\t \$sum: $sum";

my $output = concat_w_NO_return($str1, $str2);  
say "\t \$output: $output";
[download]

Output:

         $sum: 3
         $output: foobar
[download]

P.S.: perldoc -f return puts the explanation this way:

(In the absence of an explicit "return", a subroutine, eval, or do FILE automatically returns the value of the last expression evaluated.)