my @sorted =
   ((sort {$order{$a} <=> $order{$b}}
   grep {exists $order{$_}} @to_be_ordered),
   grep {not exists $order{$_}} @to_be_ordered);

##</code><code>##

my (@exists, @not_exists);
push @{exists $order{$_} ? \@exists : \@not_exists }, $_ for @to_be_ordered;
my @sorted = ((sort {$order{$a} <=> $order{$b}} @exists), @not_exists);