my @sortedarray = sort @unsortedarray;

##</code><code>##

my @sortedarray = @unsortedarray[sort @unsortedarray];
##</code><code>##

my @sorted = @unsorted[ map {$$_[1]} 
                        sort {$$b[0]<=>$$a[0]}
                        map [expensive($unsorted[$_]), $_]; } 
                        0..$#unsorted ];