if ( $si >= $sn ) {
   $spair{$si."-".$sn} = $prod;
} else {
   $spair{$sn."-".$si} = $prod;
}

##</code><code>##

use strict;
use warnings;
use Data::Dumper;

my %hash2 =(
           '1' => 2,
           '3' => 5,
           '4' => 2,); 

my %spair = ();
my $prod;
foreach my $si (keys %hash2) {
    foreach my $sn (keys %hash2) {    
        if ($si == $sn) {
            $prod = $hash2{$si};
        } else {
            $prod = $hash2{$si}*$hash2{$sn};	
        }
        if ( $si >= $sn ) {
            $spair{$si."-".$sn} = $prod;
        } else {
            $spair{$sn."-".$si} = $prod;
        }
    }
}
print Dumper \%spair;

##</code><code>##

$VAR1 = {
       '3-1' => '10',
       '3-3' => 5,
       '1-1' => 2,
       '4-1' => '4',
       '4-3' => '10',
       '4-4' => 2
    };