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

##</code><code>##

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

##</code><code>##

$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