#!/usr/bin/perl

use strict;
use warnings;

my @array=qw/a b c d/;

while ( @array > 1 ) {
    my $first = shift(@array);
    for my $next ( @array ) {
        # take diff $first - $next;
        # take product $first * $next;
        print sprintf("%-8s%s\n","$first - $next","$first * $next");
    }
}

exit;

__END__

a - b   a * b
a - c   a * c
a - d   a * d
b - c   b * c
b - d   b * d
c - d   c * d