user defined sort subroutine.

in the subroutine we just have to compare two variables $a and $b. you dont have to define them. just compare then and return the value as your wish. then call it via sort.

If $a should appear before $b in the final list, the sort subroutine returns −1 to say so. If $b should appear before $a, it returns 1.If the order of $a and $b doesn’t matter, the subroutine returns 0. Why would it not
matter? Perhaps you’re doing a case-insensitive sort and the two strings are fred and
Fred. Or perhaps you’re doing a numeric sort and the two numbers are equal.

my @num = qw /17 1000 04 1.50 3.14159 -10 1.5 4 2001 90210 666/;
sub my_sort{
    if($a > $b){
        1;
    }elsif($a < $b){
        -1;
    }else{
        0;
    }
}
@new = sort my_sort @num;
print "@new";

-10 1.50 1.5 3.14159 04 4 17 666 1000 2001 90210

se the spaceship operator (<=>).  This operator compares two numbers and returns
−1,  0, or  1 as needed to sort them numerically.

sub by_number { $a <=> $b }

sub by_number { $b <=> $a };
@new = sort by_number @num;
print "@new";

90210 2001 1000 666 17 04 4 3.14159 1.50 1.5 -10

 there’s a corre-sponding three-way string-comparison operator: cmp.

sub case_insensitive { "\L$a" cmp "\L$b" }

 by replacing the name of the sort routine
with the entire sort routine “inline,” like so:

my @numbers = sort { $a <=> $b } @some_numbers;