Both are the following portable and reliable. There difference is what happens when the trailing slash is ommited.

1) Anything after the last slash is considered a dir name:

my ($vol, $dirs) = File::Spec->splitpath($path, 1);
$dirs = File::Spec->canonpath($dirs);
my @dirs = File::Spec->splitdir($dirs);
pop(@dirs);
$dirs = File::Spec->catdir(@dirs);
$path = File::Spec->catpath($vol, $dirs, '');
[download]

Test:

use strict;
use warnings;

use File::Spec ();

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

   print("$path\n");

   my ($vol, $dirs) = File::Spec->splitpath($path, 1);
   $dirs = File::Spec->canonpath($dirs);
   my @dirs = File::Spec->splitdir($dirs);
   pop(@dirs);
   $dirs = File::Spec->catdir(@dirs);
   $path = File::Spec->catpath($vol, $dirs, '');

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

outputs:

/rootdir/sub1/sub2/sub3/sub4/
  -> \rootdir\sub1\sub2\sub3
/rootdir/sub1/sub2/sub3/sub4
  -> \rootdir\sub1\sub2\sub3
/rootdir/
  -> \
/rootdir
  -> \
/
  -> \
C:\
  -> C:\
reldir/sub1/
  -> reldir
reldir/sub1
  -> reldir
reldir/
  ->
reldir
  ->
[download]

2) Anything after the last slash is considered a file name:

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, '');
[download]

Test:

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");
}
[download]

outputs:

/rootdir/sub1/sub2/sub3/sub4/
  -> \rootdir\sub1\sub2\sub3
/rootdir/sub1/sub2/sub3/file
  -> \rootdir\sub1\sub2
/rootdir/
  -> \
/rootfile
  -> \
/
  -> \
C:\
  -> C:\
reldir/sub1/
  -> reldir
reldir/file
  ->
reldir/
  ->
relfile
  ->
[download]

Note: The only difference is the second argument to splitpath.