my %hash1;
my %hash2 = (1 => 2,
             3 => 5,
             4 => 2,);

my @keys = keys %hash2;

for (my $i=0; $i<@keys; $i++) {     
    $hash1{"$keys[$i]-$keys[$i]"}=$hash2{$keys[$i]};  
    for (my $j=$i+1; $j<@keys; $j++) {       
        $hash1{"$keys[$i]-$keys[$j]"} = $hash2{$keys[$i]}*$hash2{$keys[$j]};  
    }
}