use strict;
use warnings;

use File::Spec ();

foreach (
   '/rootdir/sub1/sub2/sub3/sub4/',
   '/rootdir/sub1/sub2/sub3/file',
   '/rootdir/',
   '/rootfile',
   '/',
   'C:\\',
   'reldir/sub1/',
   'reldir/file',
   'reldir/',
   'relfile',
) {
   my $path = $_;

   print("$path\n");

   my ($vol, $dirs) = File::Spec->splitpath($path, 0);
   $dirs = File::Spec->canonpath($dirs);
   my @dirs = File::Spec->splitdir($dirs);
   pop(@dirs);
   $dirs = File::Spec->catdir(@dirs);
   $path = File::Spec->catpath($vol, $dirs, '');

   print("  -> $path\n");
}