$dat{"./file"} = 1;
$dat{"./a"} = 2;
$dat{"./a/ab"} = 3;
$dat{"./a/a"} = 4;
$dat{"./aa/ba"} = 5;

foreach (sort by_dir keys %dat) {
   print "dat = $dat{$_} for $_\n";
}

sub by_dir {
   local ($aa, $bb, $na, $nb);
   $na = $nb = 0;
   $aa = $a;
   $bb = $b;
   while ($aa =~ s/\///) {
     $na += 1;
   }
   while ($bb =~ s/\///) {
     $nb += 1;
   }
   if ($na > $nb) {
      1;
   } elsif ($na < $nb) {
      -1;
   } else {
     $a cmp $b;
   }
}