You don't really need a loop for that, you could do it like this:

mounted_disks=$(mount | perl -le'$d = shift; print join " ", sort { le
+ngth( $b ) <=> length( $a ) } map m{/dev/$d} ? (split)[2] : (), <>' $
+dest)
[download]

Or even move the mount command into the perl code:

mounted_disks=$(perl -le'$d = shift; print join " ", sort { length( $b
+ ) <=> length( $a ) } map m{/dev/$d} ? (split)[2] : (), qx(mount)' $d
+est)
[download]