You could do something like this, but it is certainly no easier or clearer than two nested loops:

for my $keys ( map{ my $k=$_; map[ $k, $_ ],keys %{ $h{ $k}}  } keys %
+h ) { 
    print "@$keys : ", $h{ $keys->[0] }{ $keys->[1] }; 
};;
a a7 :  7
a a9 :  9
a a8 :  8
a a3 :  3
a a10 :  10
a a2 :  2
a a4 :  4
a a5 :  5
a a6 :  6
a a1 :  1
b b6 :  6
b b7 :  7
b b4 :  4
b b8 :  8
b b9 :  9
b b3 :  3
b b10 :  10
b b1 :  1
b b2 :  2
b b5 :  5
...
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If you need to process a variable depth structure, then you might look at Data::Rmap or Data::Diver, but you may find them unsatisfying difficult to use.

Every time I've tried to use the former, I've ended up reverting to a recursive subroutine tailored to my particular use.