#$hash{$row}{$cols[$i+1]} = $vals[$i]; #toolic's
    $hash{"$row,$cols[$i+1]"} = $vals[$i]; #1-D version

##</code><code>##

foreach (sort keys %hash)
{
   print "$_  $hash{$_}\n";
}
__END__
prints:
aaa,bar  456
aaa,baz  789
aaa,foo  123
bbb,bar  654
bbb,baz  321
bbb,foo  987