Try Win32::DriveInfo. The DrivesInUse routine returns an array of all drive-letters in use.

Update: I ran this little test on my system

use strict;
use warnings;

use Win32::DriveInfo;

my @drives = Win32::DriveInfo::DrivesInUse();

print join( "\n", @drives );
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A
C
D
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use Win32::OLE qw( in );

my $wmi_service = Win32::OLE->GetObject("winmgmts:{impersonationLevel=
+impersonate}!\\\\.\\root\\cimv2")
   or die("Unable to connect to WMI: $!\n");

# DriveType 3 = local hard disk
my $disk_col = $wmi_service->ExecQuery("
   SELECT DeviceID
     FROM Win32_LogicalDisk
    WHERE DriveType = 3
");

my @drives = map { $_->DeviceID } in $disk_col;

local $, = ", ";
local $\ = "\n";
print @drives;
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On my computer, the above prints C:, D:, E:.
If I remove the WHERE clause, it prints A:, B:, C:, D:, E:, F:, G:, H: (Floppy drives, hard drives, optical drives, USB drives, network drives, virtual drives, etc.)

You might also wish to have a look at the Physical and Logical Disks (Perl Scripts) section of the MS Script Center Repository.

HTH,

sub drives {
    my @drives = ();
    eval{require Win32API::File;};
    return map {"$_:\\"} ('C' .. 'Z') if $@;
    my @r = Win32API::File::getLogicalDrives();
    return unless @r > 0;
    for (@r) {
        my $t = Win32API::File::GetDriveType($_);
        push @drives, $_ if ($t == 3 or $t == 4);
    }
    return @drives > 0 ? @drives : undef;
}
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