Here's another version that might be easier for you to understand:

my @array_of_references;

for my $line (<DATA>) {
    my @values = split ' ', $line;
    push @array_of_references, \@values;  #The \ creates a reference.
}

my @sorted_refs = sort  by_my_custom_func   @array_of_references;

sub by_my_custom_func {

    if ($a->[0] < $b->[0]) {return -1}     # $a should come before $b
    elsif ($a->[0] > $b->[0]) {return +1}  # $a should come after $b

    #Execution arrives here only if $a->[0] equals
    #$b->[0]:

    if ($a->[1] < $b->[1]) {return +1}     # $a should come after $b
    elsif ($a->[1] > $b->[1]) {return -1}  # $a should come before $b
    else {return 0}

}

for my $arr_ref (@sorted_refs){
    print "@$arr_ref" . "\n";}

__DATA__
7 22
12 20
7 15
1 5
7 10


--output:--
1 5
7 22
7 15
7 10
12 20
[download]

perl's sort function can be a little hard to understand. As far as I can tell, the value you return from a custom sort function always determines what happens to $a. If you return a negative number, then $a will come before $b in the sorted results; and if you return a positive number then $a will come after $b in the sorted results.

Note that the <=> operator is just a shortcut for:

if ($a < $b) {return -1}
elseif ($a > $b) {return +1}
else {return 0}
[download]

However if you reverse the order of $a and $b, e.g. $b <=> $a, you get a reverse sort. That's because when sort calls the <=> operator, otherwise known as the "spaceship operator", sort calls it like this:

$a = 1;
$b = 2;

my $result1 = spaceship($a, $b);
print $result1 . "\n";

my $result2 = spaceship($b, $a);
print $result2 . "\n";


sub spaceship {
   my $x = shift;
   my $y = shift;

   if ($x < $y) {return -1}
   elsif ($x > $y) {return +1}
   else {return 0};
}

--output:--
-1 
1
[download]

Remember, sort() uses the return value from the sort function, in this case <=>, to determine what to do with $a. In the first case, $a <=> $b, the -1 tells sort() that $a should go before $b, and in the second case, $b <=> $a, the 1 tells sort() that $a goes after $b. In short, the <=> operator always thinks what is on its right is $a and what's on its left is $b.

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