If you know the input will be of that form [as opposed to
c:\dir1\subdir1 (lowercase C:),
\dir1\subdir1 (no drive),
dir1\subdir1 (relative path) or
C:/dir1/subdir1 (alternate slashes)
], the following will do the trick:

my ($subdir1) = /^C:\\[^\\]+\\([^\\]+)/;
[download]

Otherwise, use core module File::Spec. (I don't see how File::Basename would help.)

use File::Spec ();
my ($subdir1) = (File::Spec->splitdir(File::Spec->rel2abs($_)))[2];
[download]