my %children = (
                  'a' => [ 'b', 'c' ],
                  'c' => [ 'x' ],
                  'b' => [ 'x' ],
                  'x' => [ 'y' ],
                  'y' => [ 'z' ],
                  'z' => [ ],
                  );
sub children { @{$children{$_[0]} || []}; } 
my @unsorted = ( 'z', 'a', 'x', 'c', 'b', 'y' );
my @sorted = toposort(\&children, \@unsorted);