$VAR1 = {
          'a' => [
                   0,
                   1,
                   2
                 ],
          'b' => [
                   -1,
                   3,
                   4
                 ],
          'c' => [
                   4,
                   5,
                   6
                 ]
        };

##</code><code>##

sort { $hash{$a}[0] <=> $hash{$b}[0]} keys %hash