Your first example (changing the current drive only) looks good, but I'm not sure your second example (changing the current directory of a drive):

my $drive = substr(getcwd(), 0, 2);
chdir('C:\\');
chdir($drive);
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quite works.

Yes, it will work if you're going to the top-level directory "\", (for which you could do the simpler chdir('C:/')) but if you need to change to a directory on (for example) drive D: which doesn't exist on drive C:, then changing to C:\dir_on_drive_d will fail:

use strict;
use warnings;

use Cwd qw(getcwd chdir getdcwd);

# Here, D:\testdir exists, but C:\testdir does NOT
my $drive = substr(getcwd(), 0, 2);
chdir('C:/testdir') or die "No such directory 'C:/testdir'\n";
chdir($drive);

__END__
Produces:
No such directory 'C:/testdir'
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How about just using the drive and directory together, which seems to work okay:

use strict;
use warnings;

use Cwd qw(chdir getdcwd);

show_drives();
system("dir/w");
chdir("D:/testdir") or die "Can't change to dir 'D:/testdir' ($!)\n";
system("dir/w");

sub show_drives {
    printf "You are in '%s' on the current drive\n", getdcwd();
    printf "You are in '%s' on the 'D' drive\n",     getdcwd('D');
    print  "\n";
}
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which correctly displays the contents of D:\testdir after the command chdir("D:/testdir"):

You are in 'C:\Documents and Settings\liverpole\perl' on the current d
+rive
You are in 'D:\testdir' on the 'D' drive

 Volume in drive C has no label.
 Volume Serial Number is E451-D99E

 Directory of C:\Documents and Settings\liverpole\perl

[.]     [..]    dirtest.pl    dirtest.pl~
               2 File(s)            836 bytes
               2 Dir(s)  10,946,477,056 bytes free
 Volume in drive D is WIN2000
 Volume Serial Number is EEE2-EEE2

 Directory of D:\testdir

[.]     [..]    a.txt   b.txt   c.txt
               3 File(s)             24 bytes
               2 Dir(s)   3,777,839,104 bytes free
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