Use splitpath from File::Spec and split:

use strict;
use warnings;
use File::Spec;

while (<DATA>)
    {
    chomp;
    my @subs = split /\\/, (File::Spec->splitpath ($_)) [1];
    print "sub dir is: $subs[2]\n" if $#subs > 1;
    print "No sub dir in $_\n" if $#subs <= 1;
    }
    
__DATA__
C:\dir1\
C:\dir1\subdir1\
C:\dir1\subdir1\subdir2\
[download]

Prints:

No sub dir in C:\dir1\
sub dir is: subdir1
sub dir is: subdir1
[download]