my @sorted = 
           map{ $_->[0] }
           sort {$a->[1] cmp $b->[1]}
           map { m/(.*?)\s*(\S+)$/; [$_, "$2 $1" ] }
           @unsorted;

##</code><code>##

my @output =
  map { $_->[1] }
  sort { $a->[0] cmp $b->[1] }
  map { [$_, expensive_func($_)] }
  @input;