my @inkeys = map
                 {
                    # extract the original key name from the sorted
                    # array refs, and return them in a list to @inkeys
                    $_->[2];
                 }

                 # sort the array refs: first sort alphabetically
                 # (cmp) on the first element, then numerically
                 # (<=>) on the second element
                 sort { $a->[0] cmp $b->[0] || $a->[1] <=> $b->[1] }

                 # 'map' passes a list of array refs to 'sort'
                 map
                 {
                    # split the key name into alpha and numeric parts
                    my ( $alf, $num ) = $_ =~ m/([a-z]+)(\d+)/i;

                    # create an array ref with the two parts of the
                    # key that will be used for sorting, and the
                    # original key name, return the array ref
                    [ $alf, $num, $_ ];
                 } keys %{ $href };